Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended	6 Months Ended		12 Months Ended	18 Months Ended
	May 31, 2016	Mar. 31, 2016	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Jun. 30, 2018	Feb. 28, 2017
Stockholders' equity
Proceeds from issuance of common stock				$ 45,125	$ 76,218
Payments of stock issuance costs				784	$ 1,427
ATM Program, remaining amount authorized for issuance			$ 175,500	175,500			$ 175,500
Net proceeds from sale of common stock				$ 44,341
Maximum | Share repurchase program
Stockholders' equity
Repurchase Program, maximum amount authorized for repurchase								$ 300,000
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
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			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	$ 25.00			$ 25.00
Cumulative Redeemable Preferred Stock | Share repurchase program
Stockholders' equity
Repurchase Program, number of shares repurchased (in shares)			0	0
Common Stock | Share repurchase program
Stockholders' equity
Repurchase Program, number of shares repurchased (in shares)			0	0
Common Stock | At The Market
Stockholders' equity
Proceeds from issuance of common stock				$ 124,500
ATM Program, number of shares sold or issued (in shares)			2,590,854				7,467,709
Payments of stock issuance costs				$ 2,300
Common Stock | Maximum | At The Market
Stockholders' equity
ATM Program, maximum amount authorized for issuance								$ 300,000