Stockholders' Equity (Details) (USD $)	0 Months Ended																		1 Months Ended												12 Months Ended				12 Months Ended					124 Months Ended	0 Months Ended	112 Months Ended	12 Months Ended				0 Months Ended	12 Months Ended			116 Months Ended	0 Months Ended		3 Months Ended		12 Months Ended
	Dec. 11, 2013		Sep. 26, 2013	Aug. 01, 2013		Jun. 28, 2013	Mar. 28, 2013	Mar. 04, 2013		Dec. 12, 2012	Sep. 28, 2012	Jun. 27, 2012	Mar. 23, 2012	Dec. 14, 2011		Sep. 26, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Oct. 30, 2012	Jul. 30, 2012	Apr. 29, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Aug. 31, 2005	Dec. 31, 2013 Public Offering of Common Stock		Dec. 31, 2013 DRSPP	Dec. 31, 2012 DRSPP	Dec. 31, 2011 DRSPP	Dec. 31, 2013 DRSPP	Dec. 12, 2008 Controlled Equity Offering Program	Dec. 31, 2013 Controlled Equity Offering Program	Dec. 31, 2013 Series A Preferred Stock	Dec. 31, 2012 Series A Preferred Stock	Dec. 31, 2013 Series B Preferred Stock	Dec. 31, 2012 Series B Preferred Stock	May 16, 2013 Preferred Stock [Member] Series A Preferred Stock	Dec. 31, 2013 Preferred Stock [Member] Series A Preferred Stock	Dec. 31, 2012 Preferred Stock [Member] Series A Preferred Stock	Dec. 31, 2011 Preferred Stock [Member] Series A Preferred Stock	May 16, 2013 Preferred Stock [Member] Series A Preferred Stock	Jul. 02, 2013 Preferred Stock [Member] Series B Preferred Stock	Apr. 15, 2013 Preferred Stock [Member] Series B Preferred Stock	Dec. 31, 2013 Preferred Stock [Member] Series B Preferred Stock	Sep. 30, 2013 Preferred Stock [Member] Series B Preferred Stock	Dec. 31, 2013 Preferred Stock [Member] Series B Preferred Stock director	Dec. 31, 2012 Preferred Stock [Member] Series B Preferred Stock	Dec. 31, 2011 Preferred Stock [Member] Series B Preferred Stock
Dividends on Preferred Stock
Preferred stock, dividend rate (as a percent)																																8.50%											8.50%	8.50%	7.50%	0.00%		8.50%								7.50%
Redemption of Series A Preferred Stock (in shares)																																															3,840,000	3,840,000	0	0
Preferred stock, aggregate redemption price																																															$ 97,000,000	$ 38,000	$ 0	$ 0
Preferred stock, redemption price (in dollars per share)																																															$ 25.27153				$ 25.27153
Preferred stock, amount redemption value exceeds carrying value																																															3,900,000				3,900,000
Preferred stock, cash dividends (in dollars per share)																			$ 0.53125	$ 0.53125	$ 0.53125	$ 0.53125	$ 0.53125	$ 0.53125	$ 0.53125	$ 0.53125	$ 0.53125	$ 0.53125	$ 0.53125	$ 0.53125																	$ 0.27153				$ 0.53125	$ 0.39583		$ 0.46875	$ 0.46875
Stock reclassified during period (in shares)																																																					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	$ 25.00								$ 25.00	$ 25.00		$ 25.00
Proceeds from sale of Series B preferred stock																															200,000,000	0	0																				193,300,000
Preferred stock, voting rights, available if company fails to pay dividend, minimum term																																																								1 year 6 months
Number of additional directors that can be elected by preferred stock holders																																																								2
Minimum percentage of preferred shareholders required for approval																															66.67%
Dividends on Common Stock
Common stock, cash dividends declared (in dollars per share)	$ 0.20	[1]	$ 0.22	$ 0.28	[2]	$ 0.22	$ 0.22	$ 0.50	[3]	$ 0.200	$ 0.210	$ 0.230	$ 0.240	$ 0.270	[4]	$ 0.250	$ 0.250	$ 0.235
Common Stock, Special Dividends Per Share Declared														$ 0.02
Common stock, accrued dividends and DERs payable																															73,600,000
Shares issued																																			74,750,000	[5]	9,511,739	1,977,165	404,516	26,039,764		30,144,815														8,000,000	0	0
Gross Proceeds Per Share																																			$ 8.10	[5]
Gross Proceeds																															77,625,000	13,038,000	607,803,000		605,475,000	[5],[6]
Expenses incurred in connection with equity offering																																			438,000
Common stock, shares authorized																			895,000,000												886,950,000	895,000,000					15,000,000			15,000,000
Shares of common stock authorized and available for issuance																																					11,500,000			11,500,000
Net proceeds																																					77,600,000	15,500,000	2,800,000	221,400,000		194,900,000
Payments made for preferred stock offering costs																															(6,684,000)	0	0									(4,200,000)
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																															10,000,000.0			4,000,000.0
Number of shares repurchased through the Company's publicly announced stock repurchase program																															2,143,354	1,240,291
Value of shares repurchased through the Company's publicly announced stock repurchase program (in dollars)																															$ 15,400,000	$ 9,700,000
Average cost per share (in dollars per share)																															$ 7.20	$ 7.83
Number of Remaining shares authorized to be repurchased under the Repurchase Program (in shares)																															6,616,355

[1] At December 31, 2013, the Company had accrued dividends and DERs payable of $73.6 million related to the common stock dividend declared on December 11, 2013.

[2] Reflects the special cash dividend on common stock declared on August 1, 2013.

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

[4] Includes a special dividend of $0.02 per share.

[5] The Company did not issue any shares of common stock through public offerings during the years ended December 31, 2013 and 2012.

[6] The Company incurred approximately $438,000 of expenses in connection with this equity offering.