Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized	100,000,000	100,000,000
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	500,000,000	500,000,000
Common stock, issued	93,811,172	93,525,469
Common stock, outstanding	93,811,172	93,525,469
7.875% Series B Preferred Stock
Preferred stock, shares authorized	3,000,000
Preferred stock, shares issued	3,000,000	3,000,000
Preferred stock, shares outstanding	3,000,000	3,000,000
Preferred stock, aggregate liquidation preference (in dollars)	$ 75,509	$ 75,509
Preferred stock, dividend rate (as a percent)	7.875%	7.875%
7.125% Series C Preferred Stock
Preferred stock, shares authorized	3,400,000
Preferred stock, shares issued	3,400,000	3,400,000
Preferred stock, shares outstanding	3,400,000	3,400,000
Preferred stock, aggregate liquidation preference (in dollars)	$ 85,522	$ 85,522
Preferred stock, dividend rate (as a percent)	7.125%	7.125%
6.450% Series D Preferred Stock
Preferred stock, shares authorized	3,000,000
Preferred stock, shares issued	3,000,000	3,000,000
Preferred stock, shares outstanding	3,000,000	3,000,000
Preferred stock, aggregate liquidation preference (in dollars)	$ 75,417	$ 75,417
Preferred stock, dividend rate (as a percent)	6.45%	6.45%