Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued (in shares)	98,351,217	93,219,439
Common stock, shares outstanding	98,351,217	93,219,439
Preferred Stock [Member]
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized	30,000,000	30,000,000
Series G Cumulative Redeemable Preferred Stock [Member]
Preferred stock, par value (in dollars per share)	$ 0	$ 0.01
Preferred stock, shares authorized	0	4,000,000
Preferred stock, shares issued	0	4,000,000
Preferred stock, shares outstanding	0	4,000,000
Preferred stock dividend rate percentage	0.00%	6.875%
Preferred stock liquidation preference	$ 0	$ 100,000,000
Series H Cumulative Redeemable Preferred Stock [Member]
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized	4,000,000	4,000,000
Preferred stock, shares issued	4,000,000	4,000,000
Preferred stock, shares outstanding	4,000,000	4,000,000
Preferred stock dividend rate percentage	6.375%	6.375%
Preferred stock liquidation preference	$ 100,000,000	$ 100,000,000