CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Parenthetical) - $ / shares		12 Months Ended
	Jul. 12, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Dividends declared per share of common stock (in dollars per share)		$ 1.2	$ 1.2	$ 1.2
7.875% Series A Cumulative Redeemable Preferred Stock
Preferred series dividends declared (in dollars per share)		$ 1.285	$ 1.969	$ 1.969
Preferred stock dividend rate, percentage		7.875%	7.875%	7.875%
7.625% Series C Cumulative Redeemable Preferred Stock
Preferred series dividends declared (in dollars per share)		$ 1.906	$ 1.906	$ 1.906
Preferred stock dividend rate, percentage		7.625%	7.625%	7.625%
7.50% Series D Cumulative Redeemable Preferred Stock
Preferred series dividends declared (in dollars per share)		$ 1.875	$ 1.875	$ 1.875
Preferred stock dividend rate, percentage		7.50%	7.50%	7.50%
7.625% Series E Cumulative Redeemable Preferred Stock
Preferred series dividends declared (in dollars per share)		$ 1.906	$ 0.953
Preferred stock dividend rate, percentage	7.625%	7.625%	7.625%	7.625%
6.95% Series F Fixed-to-Floating Rate Cumulative Redeemable Preferred Stock
Preferred series dividends declared (in dollars per share)		$ 0.724	$ 0
Preferred stock dividend rate, percentage		6.95%	6.95%	6.95%