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Stockholders' Equity (Details Textuals) $ / shares in Units, shares in Millions, $ in Millions | 12 Months Ended | 60 Months Ended |
--- | --- | --- | ---
Dec. 31, 2016 USD ($) $ / shares shares | Dec. 31, 2015 $ / shares shares | Dec. 31, 2014 shares | Dec. 31, 2008 USD ($) | Dec. 31, 2006 USD ($) | Dec. 31, 2017 | Dec. 31, 2013 shares
Class of Stock [Line Items] |  |  |  |  |  |  |
Stockholders' Equity, Reverse Stock Split | Pursuant to the authorization provided at a special meeting of the Company's stockholders held on September 16, 2015, the Company completed a 1-for-15 reverse stock split of the shares of the Company’s common stock on September 30, 2015 (the Reverse Stock Split). Refer to Note 1. "Summary of Significant Accounting Policies" for additional details surrounding the Reverse Stock Split. | Pursuant to the authorization provided at a special meeting of the Company's stockholders held on September 16, 2015, the Company completed a 1-for-15 reverse stock split of the shares of the Company’s common stock on September 30, 2015 (the Reverse Stock Split). As a result of the Reverse Stock Split, every 15 shares of issued and outstanding common stock were combined into one issued and outstanding share of Common Stock, without any change in the par value per share. |  |  |  |  |
Schedule Of Stock By Class (Textuals) [Abstract] |  |  |  |  |  |  |
Common Stock, shares authorized | 53.3 |  |  |  |  |  |
Preferred Stock, Par or Stated Value Per Share | $ / shares | $ 0.01 |  |  |  |  |  |
Common Stock, par or Stated Value Per Share | $ / shares | $ 0.01 |  |  |  |  |  |
Number of votes entitled for common stock holders | one vote per share |  |  |  |  |  |
Aggregate principal amount of debentures outstanding | $ | $ 732.5 |  |  |  |  |  |
Liquidation preference of perpetual preferred stock, per share | $ / shares | $ 1,000 |  |  |  |  |  |
Perpetual Preferred Stock annual cumulative dividend rate | 3.0875% |  |  |  |  |  |
DefinedIntervalInYearstoPayDividendsonPerpetualPreferredStock |  |  |  |  |  | five |
ElectedMembersToServeOnCompanysBoardOfDirectors | two |  |  |  |  |  |
Period for payment of dividend on perpetual preferred stock | 10 |  |  |  |  |  |
NumberOfFailedAmountOfDividendsOnPreferredStock | six |  |  |  |  |  |
Authorized amount for common stock repurchase | $ | $ 1,000.0 |  |  |  |  |  |
Treasury Stock, shares | 0.8 | 0.8 |  |  |  |  |
Cost of shares repurchased | $ | $ 299.6 |  |  | $ 199.8 | $ 99.8 |  |
Amount available for repurchase of shares under share repurchase program | $ | $ 700.4 |  |  |  |  |  |
Shares relinquished | 0.1 | 0.1 | 0.1 |  |  |  |
Common Stock, shares outstanding | 18.5 | 18.5 | 18.1 |  |  |  | 18.0
Common Stock [Member] |  |  |  |  |  |  |
Schedule Of Stock By Class (Textuals) [Abstract] |  |  |  |  |  |  |
Common Stock, shares authorized | 53.3 | 53.3 |  |  |  |  |
Common Stock, par or Stated Value Per Share | $ / shares | $ 0.01 | $ 0.01 |  |  |  |  |
Common Stock, shares outstanding | 18.5 | 18.5 |  |  |  |  |
Preferred Stock [Member] |  |  |  |  |  |  |
Schedule Of Stock By Class (Textuals) [Abstract] |  |  |  |  |  |  |
Preferred Stock, Par or Stated Value Per Share | $ / shares | $ 0.01 | $ 0.01 |  |  |  |  |
Preferred Stock, shares outstanding | 0.0 | 0.0 |  |  |  |  |
Preferred Stock, shares authorized | 10.0 | 10.0 |  |  |  |  |
Series Common Stock [Member] |  |  |  |  |  |  |
Schedule Of Stock By Class (Textuals) [Abstract] |  |  |  |  |  |  |
Common Stock, shares authorized | 40.0 | 40.0 |  |  |  |  |
Common Stock, par or Stated Value Per Share | $ / shares | $ 0.01 | $ 0.01 |  |  |  |  |
Common Stock, shares outstanding | 0.0 | 0.0 |  |  |  |  |
Perpetual Preferred Stock Member |  |  |  |  |  |  |
Schedule Of Stock By Class (Textuals) [Abstract] |  |  |  |  |  |  |
Preferred Stock, shares outstanding | 0.0 | 0.0 |  |  |  |  |
Preferred Stock, shares authorized | 0.8 | 0.8 |  |  |  |  |
Treasury Stock [Member] |  |  |  |  |  |  |
Schedule Of Stock By Class (Textuals) [Abstract] |  |  |  |  |  |  |
Treasury Stock, shares | 0.5 |  |  |  |  |  |

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