Stockholders' Equity (Details) (USD $)	0 Months Ended										3 Months Ended		6 Months Ended					3 Months Ended	6 Months Ended	118 Months Ended	1 Months Ended	107 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	0 Months Ended	3 Months Ended						6 Months Ended	3 Months Ended	0 Months Ended	3 Months Ended	6 Months Ended
	Aug. 01, 2013	Jun. 28, 2013		Mar. 28, 2013	Mar. 04, 2013		Dec. 12, 2012	Sep. 28, 2012	Jun. 27, 2012	Mar. 23, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013		Jun. 30, 2012	Dec. 31, 2012	Aug. 31, 2005	Jun. 30, 2013 DRSPP	Jun. 30, 2013 DRSPP	Jun. 30, 2013 DRSPP	Dec. 31, 2008 Controlled Equity Offering Program	Jun. 30, 2013 Controlled Equity Offering Program	Jun. 30, 2013 Series A	Dec. 31, 2012 Series A	Jun. 30, 2013 Series B	Dec. 31, 2012 Series B	May 16, 2013 Preferred Stock Series A	Jun. 30, 2013 Preferred Stock Series A	Mar. 31, 2013 Preferred Stock Series A	Dec. 31, 2012 Preferred Stock Series A	Sep. 30, 2012 Preferred Stock Series A	Jun. 30, 2012 Preferred Stock Series A	Mar. 31, 2012 Preferred Stock Series A	Jun. 30, 2013 Preferred Stock Series A	Jun. 30, 2013 Preferred Stock Series A Accrued and Unpaid Dividends	Apr. 15, 2013 Preferred Stock Series B	Jun. 30, 2013 Preferred Stock Series B	Jun. 30, 2013 Preferred Stock Series B director quarter	Apr. 12, 2013 Preferred Stock Series B
Stockholders' Equity
Preferred stock, dividend rate (as a percent)																							8.50%	8.50%	7.50%	0.00%								8.50%				7.50%
Preferred stock redeemed (in shares)																											3,840,000							3,840,000
Preferred stock, shares outstanding																							0	3,840,000	8,000,000	0		0						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)																												$ 0.53125	$ 0.53125	$ 0.53125	$ 0.53125	$ 0.53125	$ 0.53125		$ 0.27153
Shares of common stock reclassified to preferred stock																																							8,050,000
Preferred stock, shares issued																							0	3,840,000	8,000,000	0										8,000,000
Preferred stock, par value (in dollars per share)																							$ 0.01	$ 0.01	$ 0.01	$ 0										$ 0.01
Preferred stock, liquidation preference (in dollars per share)																												$ 25.00						$ 25.00		$ 25.00	$ 25.00	$ 25.00
Proceeds from issuance of preferred stock													200,000,000		0																					193,300,000
Preferred stock, dividends payable (in dollars per share)																																					$ 0.39583	$ 0.39583
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)		$ 0.22	[1]	$ 0.22	$ 0.50	[2]	$ 0.20	$ 0.21	$ 0.23	$ 0.24	$ 0.22	$ 0.23	$ 0.94	[3]	$ 0.47
Dividends Declared per Share of Common Stock, special dividend (in dollars per share)	$ 0.28												$ 0.50
Common stock, accrued dividends and DERs payable											80,400,000		80,400,000
Aggregate number of shares of common stock registered for sale under the plan											886,950,000		886,950,000			895,000,000		10,000,000	10,000,000	10,000,000
Shares of common stock authorized and available for issuance																		3,500,000	3,500,000	3,500,000
Shares issued																		3,365,102	4,499,140	21,027,165		30,144,815																8,000,000
Net proceeds																		33,244,012	40,611,206	184,400,000		194,908,570
Fees and commissions paid to Cantor													$ 6,684,000		$ 0							$ 4,189,247
Maximum number of shares that may be offered and sold through Cantor																					40,000,000
Number of shares authorized to be repurchased under the Repurchase Program																	4,000,000
Number of Remaining shares authorized to be repurchased under the Repurchase Program (in shares)											2,759,709		2,759,709

[1] At June 30, 2013, the Company had accrued dividends and DERs payable of $80.4 million related to the common stock dividend declared on June 28, 2013.

[2] Reflects the special cash dividend on common stock declared on March 4, 2013.

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