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Stockholders' Equity (Narrative) (Details) |  |  |  |  |  | 1 Months Ended | 3 Months Ended | 199 Months Ended |  |  |  |  |
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Mar. 11, 2020 $ / shares | Mar. 02, 2020 USD ($) | Feb. 28, 2020 $ / shares shares | Feb. 14, 2020 $ / shares | Apr. 15, 2013 $ / shares shares | Mar. 31, 2020 USD ($) director quarter $ / shares shares | Mar. 31, 2020 USD ($) director quarter $ / shares shares | Mar. 31, 2019 USD ($) $ / shares | Mar. 31, 2020 USD ($) $ / shares shares | Dec. 31, 2019 $ / shares shares | Oct. 15, 2019 shares | Aug. 16, 2019 USD ($) | Dec. 31, 2013 shares | Aug. 31, 2005 shares
Stockholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, cash dividends declared (in dollars per share) | $ / shares | $ 0.20 |  |  |  |  |  |  | $ 0.20 |  |  |  |  |  |
Proceeds from issuance of redeemable preferred stock | $ |  |  |  |  |  |  | $ 275,000,000 | $ 0 |  |  |  |  |  |
Dividends and dividend equivalents declared and unpaid | $ |  |  |  |  |  | $ 0 | $ 0 | $ 90,353,000 | $ 0 |  |  |  |  |
Aggregate number of shares of common stock authorized (in shares) | shares |  |  |  |  |  | 874,300,000 | 874,300,000 |  | 874,300,000 | 886,950,000 |  |  |  |
Number of shares authorized to be repurchased under the Repurchase Program (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  | 10,000,000.0 | 4,000,000.0
Number of remaining shares authorized to be repurchased under the Repurchase Program (in shares) | shares |  |  |  |  |  | 6,616,355 | 6,616,355 |  | 6,616,355 |  |  |  |  |
DRSPP |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate number of shares of common stock authorized (in shares) | shares |  |  |  |  |  |  |  |  |  |  | 9,000,000.0 |  |  |
Shares of common stock authorized and available for issuance (in shares) | shares |  |  |  |  |  | 8,800,000 | 8,800,000 |  | 8,800,000 |  |  |  |  |
Common shares issued through DRSPP (in shares) | shares |  |  |  |  |  |  | 106,949 |  | 34,485,717 |  |  |  |  |
Net proceeds from shares issued through DRSPP | $ |  |  |  |  |  |  | $ 691,979 |  | $ 287,300,000 |  |  |  |  |
Series B Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, dividend rate |  |  |  |  |  |  | 7.50% | 7.50% |  |  |  |  |  |
Preferred stock, shares issued | shares |  |  |  |  |  | 8,000,000 | 8,000,000 |  | 8,000,000 | 8,000,000 |  |  |  |
Preferred stock, par value (in dollars per share) | $ / shares |  |  |  |  |  | $ 0.01 | $ 0.01 |  | $ 0.01 | $ 0.01 |  |  |  |
Dividend declared per share, preferred stock (in dollars per share) | $ / shares |  |  |  | $ 0.46875 |  |  |  | $ 0.46875 |  |  |  |  |  |
Preferred Stock, Amount of Preferred Dividends in Arrears | $ |  |  |  |  |  |  | $ 3,800,000 |  |  |  |  |  |  |
Preferred Stock, Per Share Amounts of Preferred Dividends in Arrears | $ / shares |  |  |  |  |  |  | $ 0.46875 |  |  |  |  |  |  |
Series C Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, dividend rate |  |  |  |  |  |  | 6.50% |  |  |  |  |  |  |
Preferred stock, shares issued | shares |  |  |  |  |  | 11,000,000 | 11,000,000 |  | 11,000,000 |  |  |  |  |
Preferred stock, par value (in dollars per share) | $ / shares |  |  |  |  |  | $ 0.01 | $ 0.01 |  | $ 0.01 |  |  |  |  |
Preferred Stock, Amount of Preferred Dividends in Arrears | $ |  |  |  |  |  |  | $ 1,400,000 |  |  |  |  |  |  |
Preferred Stock, Per Share Amounts of Preferred Dividends in Arrears | $ / shares |  |  |  |  |  |  | $ 0.12639 |  |  |  |  |  |  |
Preferred Stock | Series B Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, dividend rate |  |  |  |  | 7.50% |  | 7.50% | 7.50% |  |  |  |  |  |
Preferred stock, shares issued | shares |  |  |  |  | 8,000,000.0 |  |  |  |  |  |  |  |  |
Preferred stock, par value (in dollars per share) | $ / shares |  |  |  |  | $ 0.01 |  |  |  |  |  |  |  |  |
Preferred stock, liquidation preference (in dollars per share) | $ / shares |  |  |  |  | 25.00 | $ 25.00 | $ 25.00 |  | $ 25.00 | $ 25 |  |  |  |
Preferred stock, redemption price (in dollars per share) | $ / shares |  |  |  |  | $ 25.00 |  |  |  |  |  |  |  |  |
Maximum quarters without dividends to get voting rights, in quarters | quarter |  |  |  |  |  |  | 6 |  |  |  |  |  |  |
Number of additional directors that can be elected by preferred stock holders | director |  |  |  |  |  |  | 2 |  |  |  |  |  |  |
Minimum percentage of preferred shareholders required for approval (percent) |  |  |  |  | 66.67% |  |  |  |  |  |  |  |  |
Preferred Stock | Series C Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, dividend rate |  |  | 6.50% |  |  |  |  |  |  |  |  |  |  |
Preferred stock, shares issued | shares |  |  | 11,000,000.0 |  |  |  |  |  |  |  |  |  |  |
Preferred stock, par value (in dollars per share) | $ / shares |  |  | $ 0.01 |  |  |  |  |  |  |  |  |  |  |
Preferred stock, liquidation preference (in dollars per share) | $ / shares |  |  | 25.00 |  |  |  |  |  |  |  |  |  |  |
Preferred stock, redemption price (in dollars per share) | $ / shares |  |  | $ 25.00 |  |  |  |  |  |  |  |  |  |  |
Maximum quarters without dividends to get voting rights, in quarters | quarter |  |  |  |  |  | 6 |  |  |  |  |  |  |  |
Number of additional directors that can be elected by preferred stock holders | director |  |  |  |  |  | 2 |  |  |  |  |  |  |  |
Shares reclassified to redeemable capital stock (in shares) | shares |  |  | 12,650,000 |  |  |  |  |  |  |  |  |  |  |
Proceeds from issuance of redeemable preferred stock | $ |  | $ 266,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |
At The Market |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |
At-the-market, maximum potential proceeds | $ |  |  |  |  |  |  |  |  |  |  |  | $ 400,000,000.0 |  |
Number of shares issued (in shares) | shares |  |  |  |  |  |  | 0 |  |  |  |  |  |  |
Value remaining outstanding for future offerings | $ |  |  |  |  |  | $ 390,000,000.0 | $ 390,000,000.0 |  | $ 390,000,000.0 |  |  |  |  |
LIBOR | Preferred Stock | Series C Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, basis spread on variable rate |  |  | 5.345% |  |  |  |  |  |  |  |  |  |  |

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