Earnings (Loss) per Common Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings (Loss) per Common Share
Net income (loss)	$ 25.2	$ 5.0	$ (177.5)	$ 77.4
Basic weighted-average shares	81,868,508	74,309,106	83,159,949	70,413,415
Effect of dilutive securities:
Restricted stock awards	784,207	24,319		24,319
Diluted weighted-average shares	82,652,715	74,333,425	83,159,949	70,437,734
Basic net income (loss) per share	$ 0.31	$ 0.07	$ (2.13)	$ 1.10
Diluted net income (loss) per share	$ 0.31	$ 0.07	$ (2.13)	$ 1.10