Stockholders' Equity (Details) (USD $)	3 Months Ended		12 Months Ended	1 Months Ended				3 Months Ended	12 Months Ended	3 Months Ended		1 Months Ended		3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013	Feb. 28, 2014 Maximum	Mar. 31, 2013 Series A Cumulative Redeemable Preferred Stock	Mar. 31, 2014 Series A Cumulative Redeemable Preferred Stock	Dec. 31, 2013 Series A Cumulative Redeemable Preferred Stock	Mar. 31, 2014 Series D Cumulative Redeemable Preferred Stock item	Dec. 31, 2013 Series D Cumulative Redeemable Preferred Stock	Mar. 31, 2014 Series D Cumulative Redeemable Preferred Stock Maximum	Mar. 31, 2014 Series D Cumulative Redeemable Preferred Stock Minimum item	Nov. 30, 2013 Common Stock	Feb. 28, 2013 Common Stock	Mar. 31, 2014 Common Stock
Stockholders' equity
Number of shares redeemed					7,050,000
Preferred Stock, Dividend Rate, Percentage					8.00%			8.00%	8.00%
Number of shares of stock issued								4,600,000	4,600,000
Amount paid to redeem Series A preferred stock					$ 178,600,000
Accrued dividends paid on redemption					2,300,000
Additional redemption charges					4,600,000
Preferred stock, outstanding shares						0	0	4,600,000	4,600,000
Liquidation preference (in dollars per share)								$ 25.00	$ 25.00
Future redemption price (in dollars per share)								$ 25.00
Number of days after first date of change of control within which the entity may redeem the preferred stock										120 days
Number of quarters preferred stock dividends must be in arrears before preferred stockholders have certain voting rights											6
Number of additional directors preferred stockholders will be entitled to vote for if dividends are in arrears for six or more quarterly periods								2
Number of common shares which preferred stockholders may convert their shares into upon a change of control										22,571,280
Underwriting and other costs of the offering	173,000	376,000
Net proceeds from issuance of common stock	1,219,000											270,900,000	294,900,000	1,200,000
Number of shares issued												20,000,000	25,300,000	99,460
Proceeds from stock issuance used to redeem Series A preferred stock and Series C preferred stock			279,700,000
Stock Issuance Program, Authorized Amount				$ 150,000,000
Issuance of common stock, net (in shares)												20,000,000	25,300,000	99,460
Number of shares of the underwriters' over-allotment issued													3,300,000