Stockholders' Equity (Details)						3 Months Ended		6 Months Ended		12 Months Ended	154 Months Ended
	Jun. 14, 2016 $ / shares	May 18, 2016 $ / shares	Mar. 11, 2016 $ / shares	Feb. 12, 2016 $ / shares	Apr. 15, 2013 $ / shares shares	Jun. 30, 2016 USD ($) $ / shares shares	Jun. 30, 2015 USD ($) $ / shares	Jun. 30, 2016 USD ($) quarter director $ / shares shares	Jun. 30, 2015 USD ($) $ / shares	Dec. 31, 2015 USD ($) $ / shares shares	Jun. 30, 2016 USD ($) $ / shares shares	Dec. 31, 2013 shares	Aug. 08, 2013 shares	Aug. 31, 2005 shares
Stockholders' Equity
Preferred stock, shares issued						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
Preferred stock, dividend rate								7.50%		7.50%
Maximum quarters without dividends to get voting rights, in quarters | quarter								6
Common stock, cash dividends declared (in dollars per share) | $ / shares	$ 0.20		$ 0.20			$ 0.20	$ 0.20	$ 0.40	$ 0.40
Dividends and dividend equivalents declared and unpaid | $						$ 74,584,000	$ 74,584,000	$ 74,584,000	$ 74,584,000	$ 74,575,000	$ 74,584,000
Aggregate number of shares of common stock authorized (in shares)						886,950,000		886,950,000		886,950,000	886,950,000
Number of shares authorized to be repurchased under the Repurchase Program												10,000,000.0		4,000,000.0
Repurchase of shares of common stock (in shares)								0
Number of remaining shares authorized to be repurchased under the Repurchase Program (in shares)						6,616,355		6,616,355			6,616,355
DRSPP
Stockholders' Equity
Aggregate number of shares of common stock authorized (in shares)													15,000,000
Shares of common stock authorized and available for issuance (in shares)						6,800,000		6,800,000			6,800,000
Common shares issued through DRSPP (in shares)						42,458		90,132			30,819,124
Net proceeds from shares issued through DRSPP | $						$ 294,000		$ 596,000			$ 258,900,000
Series B
Stockholders' Equity
Dividend Declared Per Share, preferred stock (in dollars per share) | $ / shares		$ 0.46875		$ 0.46875
Preferred Stock | Series B
Stockholders' Equity
Preferred stock, shares issued					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	$ 25.00		$ 25.00
Preferred stock, dividend rate					7.50%			7.50%	7.50%
Preferred stock, redemption price (in dollars per share) | $ / shares					$ 25.00
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%