Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended			3 Months Ended	12 Months Ended
	Apr. 06, 2016	Apr. 30, 2016	Mar. 31, 2016	Mar. 31, 2016	Mar. 31, 2016	Dec. 31, 2015	Feb. 28, 2014
Stockholders' equity
Proceeds from preferred stock offering					$ 115,000
Payment of preferred stock offering costs					$ 4,016
Series D Cumulative Redeemable Preferred Stock
Stockholders' equity
Preferred stock, Cumulative Redeemable Preferred Stock, dividend rate (as a percent)					8.00%	8.00%
Liquidation preference (in dollars per share)			$ 25.00	$ 25.00	$ 25.00	$ 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%
Liquidation preference (in dollars per share)			$ 25.00	$ 25.00	$ 25.00
Proceeds from preferred stock offering			$ 115,000
Payment of preferred stock offering costs			$ 4,000
Future redemption price (in dollars per share)			$ 25.00
Common Stock | At The Market
Stockholders' equity
Number of shares issued (in shares)					0	0
Stock issuance program, remaining amount			$ 128,400	$ 128,400	$ 128,400
Common Stock | Maximum | At The Market
Stockholders' equity
Stock issuance program, authorized amount							$ 150,000
Subsequent Event | Series D Cumulative Redeemable Preferred Stock
Stockholders' equity
Number of shares redeemed (in shares)	4,600,000	4,600,000
Preferred stock, Cumulative Redeemable Preferred Stock, dividend rate (as a percent)		8.00%
Preferred Stock, Redemption Price Per Share	$ 25.00	$ 25.00
Preferred Stock | Series E Cumulative Redeemable Preferred Stock
Stockholders' equity
Number of shares of preferred stock issued (in shares)					4,600,000