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Capital Structure and Per Share Data - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2017 USD ($) Class Vote $ / shares shares | Dec. 31, 2016 $ / shares shares | Dec. 31, 2015 shares
Capital Structure And Per Share Data [Line Items] |  |  |
Preferred stock shares authorized | 3,000,000 |  |
Preferred stock, issued | 0 | 0 | 0
Preferred stock, outstanding | 0 | 0 | 0
Number of classes of common stock | Class | 2 |  |
Class A Convertible Preferred Stock [Member] |  |  |
Capital Structure And Per Share Data [Line Items] |  |  |
Preferred stock shares authorized | 300,000 |  |
Preferred Stock Par Value | $ / shares | $ 0.10 |  |
Preferred stock shares redeemed | 13,801.5 |  |
Preferred stock redemption price | $ / shares | $ 1,000 |  |
Series I Preferred Stock [Member] |  |  |
Capital Structure And Per Share Data [Line Items] |  |  |
Class A Convertible Preferred Stock Shares Authorized | 100,000 |  |
Series II Preferred Stock [Member] |  |  |
Capital Structure And Per Share Data [Line Items] |  |  |
Class A Convertible Preferred Stock Shares Authorized | 100,000 |  |
Series III Preferred Stock [Member] |  |  |
Capital Structure And Per Share Data [Line Items] |  |  |
Class A Convertible Preferred Stock Shares Authorized | 100,000 |  |
Common Class A [Member] |  |  |
Capital Structure And Per Share Data [Line Items] |  |  |
Common stock, shares authorized | 100,000,000 | 100,000,000 |
Common stock, par value | $ / shares | $ 0.01 | $ 0.01 |
Common stock, number of votes per share | Vote | 1 |  |
Number of shares of class A common stock issuable against each share of class B common stock | 1 |  |
Authorized amount expend on repurchase of shares | $ | $ 695,000,000 |  |
Common stock class A, shares repurchased | 32,290,493 | 30,263,196 |
Common stock class A, share repurchase price per share | $ / shares | $ 17.76 |  |
Remaining authorized amount | $ | $ 107,700,000 |  |
Antidilutive stock options excluded in computation of diluted earnings per share |  | 200,000 | 400,000
Common Class B [Member] |  |  |
Capital Structure And Per Share Data [Line Items] |  |  |
Common stock, shares authorized | 30,000,000 | 30,000,000 |
Common stock, par value | $ / shares | $ 0.01 | $ 0.01 |
Common stock, number of votes per share | Vote | 10 |  |

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- Definition The maximum number of nonredeemable convertible preferred shares (or preferred stock redeemable solely at the option of the issuer) permitted to be issued by an entity's charter and bylaws. + References No definition available. + Details
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- Definition Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 + Details
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