Debt - Adjusted Financial Statements to Apply Adopted Guidance (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Jun. 15, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of operations
Interest expense, net				$ 0.3				$ 0.3
Income from operations before income taxes				(13.6)				(18.0)
Income tax expense/(benefit)	$ 15.4			(3.1)			$ 13.8	(5.4)
Net loss	(12.6)	$ 6.5	$ (0.8)	(10.5)	$ (0.2)	$ (1.9)	(6.9)	(12.6)
Adjustments to reconcile net (loss)/income to cash provided by operating activities:
Deferred income taxes								(5.7)
Deferred financing fees amortization								0.3
Cash provided by operating activities							41.0	1.5
Net increase in cash and cash equivalents							183.7	(2.0)
Cash and cash equivalents, beginning of period			$ 11.6			$ 16.8	11.6	16.8
Cash and cash equivalents, end of period	$ 195.3			$ 14.8			$ 195.3	$ 14.8
Net loss per weighted average share - basic (in dollars per share)	$ (0.08)			$ (0.07)			$ (0.05)	$ (0.09)
Net loss weighted average share - diluted (in dollars per share)	$ (0.08)			(0.07)			$ (0.05)	(0.09)
Balance Sheet
Deferred income taxes	$ 2.2						$ 2.2			$ 13.4
Long-term debt										274.6
Total current and long-term liabilities	$ 366.5						$ 366.5			$ 400.6
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001	$ 0.001	$ 0.001	$ 0.001	$ 0.001	$ 0.001	$ 0.001	$ 0.001	$ 0.001		$ 0.001
Common stock										$ 0.1
Capital in excess of par value of common stock										520.9
Retained earnings										60.3
Accumulated other comprehensive loss										(44.4)
Treasury stock, common; 101,131 and 99,268 shares at September 30, 2021 and December 31, 2020, respectively										(0.7)
Total equity	$ 784.5	$ 797.1	$ 535.6	$ 535.7	$ 541.7	$ 540.8	$ 784.5	$ 535.7		536.2	$ 544.0
Total liabilities and equity	1,151.0						1,151.0			936.8
Statement of cash flows
Net loss	(12.6)	$ 6.5	(0.8)	(10.5)	$ (0.2)	(1.9)	(6.9)	(12.6)
Accounting Standards Update 2020-06
Statement of operations
Interest expense, net	0.0			0.3			0.3	0.3
Income from operations before income taxes	2.8			(13.6)			6.9	(18.0)
Income tax expense/(benefit)	15.4			(3.1)			13.8	(5.4)
Net loss	$ (12.6)			(10.5)			(6.9)	(12.6)
Adjustments to reconcile net (loss)/income to cash provided by operating activities:
Deferred income taxes							11.2	(5.7)
Deferred financing fees amortization							0.3	0.3
Cash provided by operating activities							$ 4.6	(18.0)
Cash and cash equivalents, beginning of period						16.8		16.8
Cash and cash equivalents, end of period				$ 14.8				$ 14.8
Net loss per weighted average share - basic (in dollars per share)	$ (0.08)			$ (0.07)			$ (0.05)	$ (0.09)
Net loss weighted average share - diluted (in dollars per share)	$ (0.08)						$ (0.05)
Balance Sheet
Deferred income taxes	$ 7.8						$ 7.8			5.6
Long-term debt	240.1						240.1			274.6
Total current and long-term liabilities	247.9						247.9			280.2
Common stock	0.1						0.1			0.1
Capital in excess of par value of common stock	776.4						776.4			520.9
Retained earnings	53.4						53.4			60.3
Accumulated other comprehensive loss	(44.6)						(44.6)			(44.4)
Treasury stock, common; 101,131 and 99,268 shares at September 30, 2021 and December 31, 2020, respectively	(0.8)						(0.8)			(0.7)
Total equity	784.5						784.5			536.2
Total liabilities and equity	1,032.4						1,032.4			816.4
Statement of cash flows
Net loss	(12.6)			$ (10.5)			(6.9)	$ (12.6)
As computed under ASC 470-20
Statement of operations
Interest expense, net	1.8			2.0			5.5	2.0
Income from operations before income taxes	1.0			(15.3)			1.7	(19.7)
Income tax expense/(benefit)	15.0			(3.5)			12.7	(5.8)
Net loss	(14.0)			(11.8)			(11.0)	(13.9)
Adjustments to reconcile net (loss)/income to cash provided by operating activities:
Deferred income taxes							10.1	(6.1)
Deferred financing fees amortization							5.5	2.0
Cash provided by operating activities							4.6	(18.0)
Net increase in cash and cash equivalents							183.7	(2.0)
Cash and cash equivalents, beginning of period			11.6			16.8	11.6	16.8
Cash and cash equivalents, end of period	$ 195.3			$ 14.8			$ 195.3	$ 14.8
Net loss per weighted average share - basic (in dollars per share)	$ (0.09)			$ (0.08)			$ (0.07)	$ (0.10)
Balance Sheet
Deferred income taxes	$ 15.0						$ 15.0			13.9
Long-term debt	207.4						207.4			236.7
Total current and long-term liabilities	222.4						222.4			250.6
Common stock	0.1						0.1			0.1
Capital in excess of par value of common stock	808.6						808.6			553.1
Retained earnings	46.7						46.7			57.7
Accumulated other comprehensive loss	(44.6)						(44.6)			(44.4)
Treasury stock, common; 101,131 and 99,268 shares at September 30, 2021 and December 31, 2020, respectively	(0.8)						(0.8)			(0.7)
Total equity	810.0						810.0			565.8
Total liabilities and equity	1,032.4						1,032.4			816.4
Statement of cash flows
Net loss	(14.0)			$ (11.8)			(11.0)	$ (13.9)
Effect of change | Accounting Standards Update 2020-06
Statement of operations
Interest expense, net	(1.8)			(1.7)			(5.2)	(1.7)
Income from operations before income taxes	1.8			1.7			5.2	1.7
Income tax expense/(benefit)	0.4			0.4			1.1	0.4
Net loss	1.4			1.3			4.1	1.3
Adjustments to reconcile net (loss)/income to cash provided by operating activities:
Deferred income taxes							1.1	0.4
Deferred financing fees amortization							(5.2)	(1.7)
Cash provided by operating activities							0.0	0.0
Net increase in cash and cash equivalents							0.0	0.0
Cash and cash equivalents, beginning of period			$ 0.0			$ 0.0	0.0	0.0
Cash and cash equivalents, end of period	$ 0.0			$ 0.0			$ 0.0	$ 0.0
Net loss per weighted average share - basic (in dollars per share)	$ 0.01			$ 0.01			$ 0.02	$ 0.01
Balance Sheet
Deferred income taxes	$ (7.2)						$ (7.2)			(8.3)
Long-term debt	32.7						32.7			37.9
Total current and long-term liabilities	25.5						25.5			29.6
Common stock	0.0						0.0			0.0
Capital in excess of par value of common stock	(32.2)						(32.2)			(32.2)
Retained earnings	6.7						6.7			2.6
Accumulated other comprehensive loss	0.0						0.0			0.0
Treasury stock, common; 101,131 and 99,268 shares at September 30, 2021 and December 31, 2020, respectively	0.0						0.0			0.0
Total equity	(25.5)						(25.5)			(29.6)
Total liabilities and equity	0.0						0.0			$ 0.0
Statement of cash flows
Net loss	$ 1.4			$ 1.3			$ 4.1	$ 1.3