Stockholders' Equity (Details) (USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended		3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2015	Feb. 28, 2014
Series D Cumulative Redeemable Preferred Stock
Stockholders' equity
Preferred stock, outstanding shares	4,600,000	4,600,000	4,600,000
Number of shares of stock issued	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
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
Number of shares issued			0
Common Stock | Maximum | At The Market
Stockholders' equity
Stock issuance program, authorized amount				$ 150.0