Summary of Significant Accounting Policies - Earnings or Loss per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings or Loss per Share
Net income (loss)	$ 84.2	$ (2.1)	$ 5.0	$ 72.4	$ 28.1	$ (20.3)	$ 14.2	$ 4.3	$ 159.5	$ 26.3	$ (48.7)
Basic weighted-average shares									78,778,640	65,837,555	63,717,617
Effect of dilutive securities:
Restricted stock awards									137,306	26,648
Diluted weighted-average shares									78,915,946	65,864,203	63,717,617
Basic net income (loss) per share (in dollars per share)	$ 0.97	$ (0.02)	$ 0.07	$ 1.09	$ 0.42	$ (0.31)	$ 0.21	$ 0.07	$ 2.03	$ 0.40	$ (0.76)
Diluted net income (loss) per share (in dollars per share)	$ 0.97	$ (0.02)	$ 0.07	$ 1.09	$ 0.42	$ (0.31)	$ 0.21	$ 0.07	$ 2.02	$ 0.40	$ (0.76)