Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		9 Months Ended		12 Months Ended
	May 31, 2016	Mar. 31, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Feb. 28, 2017
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
Payments of Stock Issuance Costs			1,475
Preferred stock redemption charge				$ 4,052
Net proceeds from sale of common stock			$ 77,932
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
Proceeds from preferred stock offerings		$ 115,000
Payment of preferred stock offering costs		$ 4,000
Redemption price (in dollars per share)			$ 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
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
Payments of Stock Issuance Costs			$ 1,500
ATM Program, number of shares sold or issued (in shares)			4,876,855
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