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Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) (USD $)
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Jun. 30, 2013
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Dec. 31, 2012
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| Convertible preferred stock issued | 0 | 0 |
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Common Class A
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| Common stock, par value | $ 0.01 | $ 0.01 |
| Common stock, shares authorized | 100,000,000 | 100,000,000 |
| Common stock, shares issued | 61,535,416 | 61,352,134 |
| Common stock, shares outstanding | 40,767,025 | 41,210,507 |
| Treasury stock, shares | 20,768,391 | 20,141,627 |
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Common Class B
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| Common stock, par value | $ 0.01 | $ 0.01 |
| Common stock, shares authorized | 30,000,000 | 30,000,000 |
| Common stock, shares issued | 12,029,375 | 12,029,375 |
| Common stock, shares outstanding | 12,029,375 | 12,029,375 |
| X | ||||||||||
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- Definition
Face amount or stated value per share of common stock. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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| X | ||||||||||
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- Definition
The maximum number of common shares permitted to be issued by an entity's charter and bylaws. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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| X | ||||||||||
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- Definition
Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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| X | ||||||||||
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- Definition
Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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| X | ||||||||||
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- Definition
Total number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) issued to shareholders (includes related preferred shares that were issued, repurchased, and remain in the treasury). May be all or portion of the number of preferred shares authorized. Excludes preferred shares that are classified as debt. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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| X | ||||||||||
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- Definition
Number of common and preferred shares that were previously issued and that were repurchased by the issuing entity and held in treasury on the financial statement date. This stock has no voting rights and receives no dividends. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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