Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2015	Jun. 30, 2015	Feb. 28, 2014
Series D Cumulative Redeemable Preferred Stock
Stockholders' equity
Number of shares of stock issued	4,600,000	4,600,000	4,600,000	4,600,000
Preferred stock, 8.0% 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
Future redemption price (in dollars per share)	$ 25.00	$ 25.00
Common Stock | At The Market
Stockholders' equity
Stock issuance program, remaining amount	$ 128.4		$ 128.4	$ 128.4
Number of shares issued			0	0
Common Stock | Maximum | At The Market
Stockholders' equity
Stock issuance program, authorized amount					$ 150.0