Stockholders' Equity (Details) (USD $)	0 Months Ended			3 Months Ended		6 Months Ended			12 Months Ended		3 Months Ended	6 Months Ended	130 Months Ended	0 Months Ended	6 Months Ended	0 Months Ended				3 Months Ended	6 Months Ended
	Jun. 13, 2014		Mar. 10, 2014	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013		Dec. 31, 2013	Aug. 31, 2005	Jun. 30, 2014 DRSPP	Jun. 30, 2014 DRSPP	Jun. 30, 2014 DRSPP	May 16, 2013 Preferred Stock Series A	Jun. 30, 2014 Preferred Stock Series A	May 19, 2014 Preferred Stock Series B	Feb. 14, 2014 Preferred Stock Series B	Jul. 01, 2013 Preferred Stock Series B	Apr. 15, 2013 Preferred Stock Series B	Jun. 30, 2014 Preferred Stock Series B	Jun. 30, 2014 Preferred Stock Series B quarter director
Stockholders' Equity
Preferred stock, dividend rate (as a percent)						7.50%			7.50%						8.50%						7.50%
Preferred stock redeemed (in shares)														3,840,000
Preferred stock, shares outstanding				8,000,000		8,000,000			8,000,000						0
Payments to redeem preferred stock														$ 97,000,000
Preferred stock, redemption price (in dollars per share)														$ 25.27153						$ 25.00	$ 25.00
Preferred stock, redemption value in excess of carrying value														3,900,000
Preferred stock, cash dividends (in dollars per share)														$ 0.27153
Shares of common stock reclassified to preferred stock																			8,050,000
Preferred stock, shares issued				8,000,000		8,000,000			8,000,000										8,000,000
Preferred stock, par value (in dollars per share)				$ 0.01		$ 0.01			$ 0.01										$ 0.01
Preferred stock, liquidation preference (in dollars per share)																			$ 25.00	$ 25.00	$ 25.00
Proceeds from issuance of preferred stock						0	200,000,000												193,300,000
Preferred stock, dividends payable (in dollars per share)																		$ 0.39583
Dividend Per Share, preferred stock																$ 0.46875	$ 0.46875
Minimum number of quarters for which failure to pay dividends on the preferred stock will give rise to voting rights																					6
Number of additional directors that can be elected by preferred stock holders																					2
Minimum percentage of preferred shareholders required for approval																				66.67%
Common stock, cash dividends declared (in dollars per share)	$ 0.20	[1]	$ 0.20	$ 0.20	$ 0.22	$ 0.40	$ 0.94	[2]
Common stock, accrued dividends and DERs payable				74,100,000		74,100,000
Aggregate number of shares of common stock registered for sale under the plan				886,950,000		886,950,000			886,950,000		15,000,000	15,000,000	15,000,000
Shares of common stock authorized and available for issuance											9,300,000	9,300,000	9,300,000
Shares issued											1,182,346	2,236,131	28,275,895
Net proceeds											$ 9,200,000	$ 16,800,000	$ 238,300,000
Number of shares authorized to be repurchased under the Repurchase Program									10,000,000.0	4,000,000.0
Repurchase of common stock, shares						0
Number of Remaining shares authorized to be repurchased under the Repurchase Program (in shares)				6,616,355		6,616,355

[1] At June 30, 2014, the Company had accrued dividends and DERs payable of $74.1 million related to the common stock dividend declared on June 13, 2014.

[2] Includes a special dividend of $0.50 per share declared on March 4, 2013.