Earnings (Loss) per Common Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings (Loss) per Common Share
Net income (loss)	$ 6.9	$ 11.4	$ 9.7	$ 8.4	$ 17.1	$ 31.4	$ 25.4	$ (161.2)	$ 36.4	$ (87.3)	$ 185.3
Basic weighted-average shares									80,713,926	81,808,425	78,778,640
Effect of dilutive securities:
Restricted stock awards									475,236		137,306
Diluted weighted-average shares									81,189,162	81,808,425	78,915,946
Basic net income (loss) per share (in dollars per share)	$ 0.08	$ 0.14	$ 0.12	$ 0.10	$ 0.21	$ 0.39	$ 0.31	$ (1.91)	$ 0.45	$ (1.07)	$ 2.35
Diluted net income (loss) per share (in dollars per share)	$ 0.08	$ 0.14	$ 0.12	$ 0.10	$ 0.21	$ 0.38	$ 0.31	$ (1.91)	$ 0.45	$ (1.07)	$ 2.35