Net (Loss) Income Per Common Share - Net (Loss) Income Per Common Share on Basic and Diluted Basis (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended							6 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net (loss) income	$ (23.1)	$ (5.4)	$ (24.7)	$ (102.7)	$ (26.0)	$ 27.0	$ 87.6	$ (30.1)	$ 114.6	$ (53.2)	$ 88.6	$ (54.1)	$ (14.1)	$ 57.0
Dividends on preferred shares												4.6	0.0	0.0
Net (loss) income attributable to common shareholders	$ (25.1)				$ (26.0)					$ (55.2)	$ 88.6	$ (58.7)	$ (14.1)	$ 57.0
Weighted average common shares outstanding - basic (in shares)												34,614,813	34,205,954	33,922,009
Effect of dilutive securities:
Equity compensation awards (in shares)												0	0	756,772
Weighted average common shares outstanding - diluted (in shares)												34,614,813	34,205,954	34,678,781
Net (loss) income per common share:
Basic (in dollars per share)	$ (0.72)	$ (0.16)	$ (0.72)	$ (2.99)	$ (0.76)	$ 0.79	$ 2,580,000	$ (0.87)	$ 3.36	$ (1.60)	$ 2.59	$ (1.70)	$ (0.41)	$ 1.68
Diluted (in dollars per share)	$ (0.72)	$ (0.16)	$ (0.72)	$ (2.99)	$ (0.76)	$ 0.78	$ 2,520,000	$ (0.87)	$ 3.30	$ (1.60)	$ 2.55	$ (1.70)	$ (0.41)	$ 1.65