Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 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, shares issued	104,287,128	93,525,469
Common stock, shares outstanding	104,287,128	93,525,469
7.875% Series B Preferred Stock
Preferred stock, shares issued		3,000,000
Preferred stock, shares outstanding		3,000,000
Preferred stock, aggregate liquidation preference (in dollars)		$ 75,509
Preferred stock, dividend rate (as a percent)		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%
6.45% 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%
6.25% Series E Preferred Stock
Preferred stock, shares authorized	6,400,000
Preferred stock, shares issued	6,400,000
Preferred stock, shares outstanding	6,400,000
Preferred stock, aggregate liquidation preference (in dollars)	$ 160,861
Preferred stock, dividend rate (as a percent)	6.25%