Stockholders' Equity (Narrative) (Details) $ / shares in Units, $ in Thousands								3 Months Ended		9 Months Ended		12 Months Ended	181 Months Ended
	Sep. 13, 2018 $ / shares	Aug. 20, 2018 $ / shares	Jun. 07, 2018 $ / shares	May 17, 2018 $ / shares	Mar. 07, 2018 $ / shares	Feb. 20, 2018 $ / shares	Apr. 15, 2013 $ / shares shares	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares	Sep. 30, 2018 USD ($) director quarter $ / shares shares	Sep. 30, 2017 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 16, 2016 shares	Dec. 31, 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 (as a percent)										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	$ 0.20	$ 0.60	$ 0.60
Dividends and dividend equivalents declared and unpaid | $								$ 90,136	$ 79,605	$ 90,136	$ 79,605	$ 79,771	$ 90,136
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 (in shares)															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)								11,900,000		11,900,000			11,900,000
Common shares issued through DRSPP (in shares)								64,731		302,264			33,978,241
Net proceeds from shares issued through DRSPP | $								$ 519		$ 2,200			$ 283,700
Series B
Stockholders' Equity
Dividend declared per share, preferred stock (in dollars per share) | $ / shares		$ 0.46875		$ 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	$ 25.00
Preferred stock, dividend rate (as a percent)							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%