Summary of Significant Accounting Policies - Earnings or Loss per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Basic and diluted earnings (loss) per common shares
Net income (loss)	$ (2.1)	$ (20.3)	$ 75.3	$ (1.8)
Basic weighted-average shares	86,973,345	66,525,044	76,014,568	65,605,874
Effect of dilutive securities:
Restricted stock awards			81,833
Diluted weighted-average shares	86,973,345	66,525,044	76,096,401	65,605,874
Basic net income (loss) per share	$ (0.02)	$ (0.31)	$ 0.99	$ (0.03)
Diluted net income (loss) per share	$ (0.02)	$ (0.31)	$ 0.99	$ (0.03)