Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			3 Months Ended	8 Months Ended	10 Months Ended	12 Months Ended
	May 31, 2016	Apr. 30, 2016	Mar. 31, 2016	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 28, 2017	Feb. 28, 2014
Stockholders' equity
Proceeds from preferred stock offerings								$ 190,000
Payment of preferred stock offering costs								6,640
Proceeds from issuance of common stock							$ 79,407	55,133
Payments of stock issuance costs							1,475	941
Preferred stock redemption charge								4,052
Net proceeds from sale of common stock							$ 77,932	$ 54,192
Share repurchase program
Stockholders' equity
Repurchase Program, maximum amount authorized for repurchase										$ 300,000
Series D Cumulative Redeemable Preferred Stock
Stockholders' equity
Preferred stock, Cumulative Redeemable Preferred Stock, dividend rate (as a percent)				8.00%					8.00%
Redemption price (in dollars per share)		$ 25.00
Number of shares redeemed (in shares)		4,600,000
Preferred stock redemption charge		$ 4,100
Series E Cumulative Redeemable Preferred Stock
Stockholders' equity
Number of shares of preferred stock sold (in shares)			4,600,000
Preferred stock, Cumulative Redeemable Preferred Stock, dividend rate (as a percent)						6.95%	6.95%	6.95%
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
Redemption price (in dollars per share)			$ 25.00	$ 25.00
Series F Cumulative Redeemable Preferred Stock
Stockholders' equity
Number of shares of preferred stock sold (in shares)	3,000,000
Preferred stock, Cumulative Redeemable Preferred Stock, dividend rate (as a percent)					6.45%		6.45%	6.45%
Liquidation preference (in dollars per share)	$ 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
Redemption price (in dollars per share)	$ 25.00
Cumulative Redeemable Preferred Stock | Share repurchase program
Stockholders' equity
Repurchase Program, number of shares repurchased (in shares)							0
Common Stock | Share repurchase program
Stockholders' equity
Repurchase Program, number of shares repurchased (in shares)							0
Common Stock | At The Market
Stockholders' equity
Proceeds from issuance of common stock							$ 79,400	$ 55,100
Payments of stock issuance costs							$ 1,500	$ 900
ATM Program, number of shares sold or issued (in shares)							4,876,855	3,564,047
ATM Program, remaining amount authorized for issuance							$ 220,600
Common Stock | Maximum | At The Market
Stockholders' equity
ATM Program, maximum amount authorized for issuance										$ 300,000	$ 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
Number of shares of preferred stock sold (in shares)								4,600,000
Preferred Stock | Series F Cumulative Redeemable Preferred Stock
Stockholders' equity
Number of shares of preferred stock sold (in shares)								3,000,000