CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Unaudited) (Parenthetical) - $ / shares		3 Months Ended		6 Months Ended		12 Months Ended
	Aug. 25, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Dividends declared per share of common stock (in dollars per share)		$ 0.3	$ 0.3	$ 0.6	$ 0.60
7.875% Series A Cumulative Redeemable Preferred Stock
Preferred Stock dividend rate, percentage	7.875%			7.875%	7.875%
Preferred series dividends declared, per share (in dollars per share)				$ 0	$ 0.984
7.625% Series C Cumulative Redeemable Preferred Stock
Preferred Stock dividend rate, percentage				7.625%	7.625%	7.625%
Preferred series dividends declared, per share (in dollars per share)				$ 0.953	$ 0.953
7.50% Series D Cumulative Redeemable Preferred Stock
Preferred Stock dividend rate, percentage				7.50%	7.50%	7.50%
Preferred series dividends declared, per share (in dollars per share)				$ 0.938	$ 0.938
7.625% Series E Cumulative Redeemable Preferred Stock
Preferred Stock dividend rate, percentage				7.625%	7.625%	7.625%
Preferred series dividends declared, per share (in dollars per share)				$ 0.196	$ 0.953
6.95% Series F Cumulative Redeemable Preferred Stock
Preferred Stock dividend rate, percentage				6.95%	6.95%	6.95%
Preferred series dividends declared, per share (in dollars per share)				$ 0.869	$ 0
6.50% Series G Cumulative Redeemable Preferred Stock
Preferred Stock dividend rate, percentage				6.50%	6.50%	6.50%
Preferred series dividends declared, per share (in dollars per share)				$ 0.763	$ 0