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Capital Structure and Per Share Data - Additional Information (Details) | 12 Months Ended |
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Dec. 31, 2018 USD ($) Class Vote $ / shares shares | Dec. 31, 2017 USD ($) $ / shares shares
Capital Structure And Per Share Data [Line Items] |  |
Preferred stock shares authorized | 3,000,000 |
Preferred stock, issued | 0 |
Number of classes of common stock | Class | 2 |
Class A convertible preferred stock |  |
Capital Structure And Per Share Data [Line Items] |  |
Preferred stock shares authorized | 300,000 |
Preferred Stock par value (in dollars per share) | $ / shares | $ 0.10 |
Preferred stock shares redeemed | 13,801.5 |
Preferred stock redemption price (in dollars per share) | $ / shares | $ 1,000 |
Series I Preferred Stock |  |
Capital Structure And Per Share Data [Line Items] |  |
Preferred stock authorized (in shares) | 100,000 |
Series II Preferred Stock |  |
Capital Structure And Per Share Data [Line Items] |  |
Preferred stock authorized (in shares) | 100,000 |
Series III Preferred Stock |  |
Capital Structure And Per Share Data [Line Items] |  |
Preferred stock authorized (in shares) | 100,000 |
Class A common stock |  |
Capital Structure And Per Share Data [Line Items] |  |
Common stock, shares authorized | 100,000,000 | 100,000,000
Common stock, par value (in dollars per share) | $ / 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 | $ |  | $ 695,000,000.0
Common stock class A, shares repurchased | 33,476,159 | 32,290,493
Common stock class A, share repurchase price per share (in dollars per share) | $ / shares | $ 17.85 |
Remaining authorized amount | $ | $ 83,600,000 |
Class B common stock |  |
Capital Structure And Per Share Data [Line Items] |  |
Common stock, shares authorized | 30,000,000 | 30,000,000
Common stock, par value (in dollars per share) | $ / 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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