Earnings (Loss) Per Share - Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended						9 Months Ended		12 Months Ended
	Jan. 19, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Basic EPS:
Net income		$ 17.0	$ 18.4	$ 25.8	$ 5.2	$ 15.1	$ 18.2	$ 61.2	$ 38.5
Less dividends on preferred shares		0.0			(5.6)			(592.3)	(16.6)
Less income allocated to preferred shareholders		0.0			0.0			0.0	(10.9)
Net income (loss) available/allocated to common shareholders - basic		17.0			(0.4)			(531.1)	11.0
Diluted EPS:
Less income allocated to preferred shareholders		0.0			0.0			0.0	(8.7)
Net income (loss) available/allocated to common shareholders - diluted		$ 17.0			$ (0.4)			$ (531.1)	$ 13.2
Basic weighted-average common shares (in shares)		278,655,269			115,976,377			262,209,929	115,164,631
Add: Effect of dilutive equity awards (in shares)		7,281,436			0			0	12,293,285
Add: Effect of dilutive warrants (in shares)		34,680,381			0			0	43,720,170
Diluted weighted average common shares (in shares)		320,617,086			115,976,377			262,209,929	171,178,086
Net income (loss) per common share (basic) (in dollars per share)		$ 0.06			$ 0			$ (2.03)	$ 0.10
Net income (loss) per common share (diluted) (in dollars per share)		$ 0.05			$ 0			$ (2.03)	$ 0.08
Redeemable Convertible Preferred Stock
Diluted EPS:
Preferred stock, dividend rate (percentage)								8.00%		8.00%
TCP-ASC ACHI Series LLLP
Basic EPS:
Less dividends on preferred shares	$ (592.3)
TCP-ASC ACHI Series LLLP | Redeemable Convertible Preferred Stock
Diluted EPS:
Preferred stock, dividend rate (percentage)	8.00%