CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	500,000,000	500,000,000
Common stock, shares issued (in shares)	62,469,535	61,723,387
Common stock, shares outstanding (in shares)	62,469,535	61,723,387
Special voting preferred shares
Preferred stock, shares issued (in shares)	21,230,769	21,230,769
Preferred stock, shares outstanding (in shares)	21,230,769	21,230,769
8.25% Series A preferred stock
Preferred stock, dividend rate (as a percent)	8.25%	8.25%
Preferred stock, aggregate liquidation preference	$ 38,788	$ 38,788
Preferred stock, shares issued (in shares)	1,551,500	1,551,500
Preferred stock, shares outstanding (in shares)	1,551,500	1,551,500
7.75% Series B preferred stock
Preferred stock, dividend rate (as a percent)	7.75%	7.75%
Preferred stock, aggregate liquidation preference	$ 31,500	$ 31,500
Preferred stock, shares issued (in shares)	1,260,000	1,260,000
Preferred stock, shares outstanding (in shares)	1,260,000	1,260,000
8.50% Series C preferred stock
Preferred stock, dividend rate (as a percent)	8.50%	8.50%
Preferred stock, aggregate liquidation preference	$ 22,500	$ 22,500
Preferred stock, shares issued (in shares)	900,000	900,000
Preferred stock, shares outstanding (in shares)	900,000	900,000