Earnings (Loss) per Common Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings (Loss) per Common Share
Net income (loss)	$ 9.4	$ 8.2	$ 24.6	$ (162.0)	$ 17.6	$ (137.4)
Basic weighted-average shares	80,675,392		81,868,508		80,513,361	83,159,949
Effect of dilutive securities:
Restricted stock awards	235,284		784,207		389,671
Diluted weighted-average shares	80,910,676		82,652,715		80,903,032	83,159,949
Basic net income (loss) per share	$ 0.12		$ 0.30		$ 0.22	$ (1.65)
Diluted net income (loss) per share	$ 0.12		$ 0.30		$ 0.22	$ (1.65)