CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) (Parenthetical)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
8.25% Series A preferred stock
Preferred stock, dividend rate (as a percent)	8.25%	8.25%	8.25%
7.75% Series B preferred stock
Preferred stock, dividend rate (as a percent)	7.75%	7.75%	7.75%
8.50% Series C preferred stock
Preferred stock, dividend rate (as a percent)	8.50%	8.50%	8.50%