Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended		3 Months Ended	4 Months Ended	6 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Mar. 11, 2016	May 31, 2016	Mar. 31, 2016	Sep. 30, 2016	Sep. 30, 2016	Sep. 30, 2016	Sep. 30, 2016	Dec. 31, 2015	Feb. 28, 2014
Stockholders' equity
Preferred stock redemption charge								$ 4,052
Proceeds from preferred stock offerings								190,000
Payment of preferred stock offering costs								6,640
Net proceeds from sales of preferred stock								$ 183,360
Series D Cumulative Redeemable Preferred Stock
Stockholders' equity
Preferred stock, Cumulative Redeemable Preferred Stock, dividend rate (as a percent)									8.00%
Preferred stock, Cumulative Redeemable Preferred Stock, shares outstanding (in shares)	0				0	0	0	0	4,600,000
Liquidation preference (in dollars per share)									$ 25.00
Series E Cumulative Redeemable Preferred Stock
Stockholders' equity
Preferred stock, Cumulative Redeemable Preferred Stock, dividend rate (as a percent)		6.95%					6.95%
Preferred stock, Cumulative Redeemable Preferred Stock, shares outstanding (in shares)	4,600,000				4,600,000	4,600,000	4,600,000	4,600,000	0
Liquidation preference (in dollars per share)	$ 25.00			$ 25.00	$ 25.00	$ 25.00	$ 25.00	$ 25.00
Proceeds from preferred stock offerings				$ 115,000
Payment of preferred stock offering costs				$ 4,000
Future redemption price (in dollars per share)	$ 25.00
Series F Cumulative Redeemable Preferred Stock
Stockholders' equity
Preferred stock, Cumulative Redeemable Preferred Stock, dividend rate (as a percent)						6.45%
Preferred stock, Cumulative Redeemable Preferred Stock, shares outstanding (in shares)	3,000,000				3,000,000	3,000,000	3,000,000	3,000,000	0
Liquidation preference (in dollars per share)	$ 25.00		$ 25.00		$ 25.00	$ 25.00	$ 25.00	$ 25.00
Proceeds from preferred stock offerings			$ 75,000
Payment of preferred stock offering costs			$ 2,600
Future redemption price (in dollars per share)	$ 25.00
Common Stock | At The Market
Stockholders' equity
Stock Issued During Period, Shares, New Issues					0			0
Number of shares issued (in shares)					0			0
Stock issuance program, remaining amount	$ 128,400				$ 128,400	$ 128,400	$ 128,400	$ 128,400
Common Stock | Maximum | At The Market
Stockholders' equity
Stock issuance program, authorized amount										$ 150,000
Preferred Stock | Series D Cumulative Redeemable Preferred Stock
Stockholders' equity
Number of shares redeemed (in shares)								(4,600,000)
Preferred Stock | Series E Cumulative Redeemable Preferred Stock
Stockholders' equity
Stock Issued During Period, Shares, New Issues								4,600,000
Number of shares issued (in shares)								4,600,000
Net proceeds from sales of preferred stock								$ 115,000
Preferred Stock | Series F Cumulative Redeemable Preferred Stock
Stockholders' equity
Stock Issued During Period, Shares, New Issues								3,000,000
Number of shares issued (in shares)								3,000,000
Net proceeds from sales of preferred stock								$ 75,000