Earnings (Loss) Per Share - Computation of Basic Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Basic EPS:
Net income (loss)	$ (23.3)	$ (8.3)
Less dividends on preferred shares	(4.6)	(4.3)
Less income allocated to preferred shareholders	0.0	0.0
Net income (loss) available/(allocated) to common shareholders - basic	(27.9)	(12.6)
Diluted EPS:
Net (loss) income	(23.3)	(8.3)
Less dividends on preferred shares	(4.6)	(4.3)
Less income allocated to preferred shareholders	0.0	0.0
Net income (loss) available/(allocated) to common shareholders - diluted	$ (27.9)	$ (12.6)
Basic weighted-average common shares (in shares)	105,831,571	101,364,424
Add: Effect of dilutive securities (in shares)	0	0
Diluted weighted-average common shares (in shares)	105,831,571	101,364,424
Net income (loss) per common share (basic) (in dollars per share)	$ (0.26)	$ (0.12)
Net income (loss) per common share (diluted) (in dollars per share)	$ (0.26)	$ (0.12)