Revision of Previously Issued Financial Statements - Condensed Consolidated Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Reclassification [Line Items]
Net income	$ 13.7	$ 42.5	$ 57.9	$ 105.6
Depreciation and amortization	13.8	13.5	42.0	44.4
Amortization of debt discount and deferred financing costs			1.3	4.5
Stock-based compensation	0.5	0.5	2.5	10.6
Retirement benefits, net			(14.1)	10.2
Loss on debt	22.7	0.9	22.7	10.5
Other, net			2.0	(0.9)
Accounts receivable			(73.4)	(9.6)
Contract assets			(49.1)	(39.1)
Other current assets			(18.2)	19.1
Recoverable environmental remediation costs			4.7	(1.8)
Other noncurrent assets			40.4	16.9
Accounts payable			(18.2)	(2.8)
Contract liabilities			(15.4)	(65.0)
Other current liabilities			116.5	2.9
Deferred income taxes			(139.2)	(3.7)
Reserves for environmental remediation costs			(3.0)	2.7
Other noncurrent liabilities and other			(33.2)	(9.1)
Net Cash (Used in) Provided by Operating Activities			(73.8)	95.4
Net Cash Used in Investing Activities			(25.2)	(19.2)
Net Cash Used in Financing Activities			(266.9)	(600.8)
Net Decrease in Cash, Cash Equivalents and Restricted Cash			(365.9)	(524.6)
Cash, Cash Equivalents and Restricted Cash at Beginning of Period			703.4	1,152.5
Cash, Cash Equivalents and Restricted Cash at End of Period	$ 337.5	627.9	$ 337.5	627.9
Previously Reported
Reclassification [Line Items]
Net income		42.6		105.7
Depreciation and amortization		13.5		44.4
Amortization of debt discount and deferred financing costs				4.5
Stock-based compensation				10.6
Retirement benefits, net				10.2
Loss on debt				10.5
Other, net				(0.9)
Accounts receivable				(9.6)
Contract assets				(39.1)
Other current assets				15.3
Recoverable environmental remediation costs				(1.8)
Other noncurrent assets				16.9
Accounts payable				(2.8)
Contract liabilities				(65.0)
Other current liabilities				2.9
Deferred income taxes				0.0
Reserves for environmental remediation costs				2.7
Other noncurrent liabilities and other				(9.1)
Net Cash (Used in) Provided by Operating Activities				95.4
Net Cash Used in Investing Activities				(19.2)
Net Cash Used in Financing Activities				(600.8)
Net Decrease in Cash, Cash Equivalents and Restricted Cash				(524.6)
Cash, Cash Equivalents and Restricted Cash at Beginning of Period				1,152.5
Cash, Cash Equivalents and Restricted Cash at End of Period		627.9		627.9
Adjustments
Reclassification [Line Items]
Net income		(0.1)		(0.1)
Depreciation and amortization		0.0		0.0
Amortization of debt discount and deferred financing costs				0.0
Stock-based compensation				0.0
Retirement benefits, net				0.0
Loss on debt				0.0
Other, net				0.0
Accounts receivable				0.0
Contract assets				0.0
Other current assets				3.8
Recoverable environmental remediation costs				0.0
Other noncurrent assets				0.0
Accounts payable				0.0
Contract liabilities				0.0
Other current liabilities				0.0
Deferred income taxes				(3.7)
Reserves for environmental remediation costs				0.0
Other noncurrent liabilities and other
Net Cash (Used in) Provided by Operating Activities				0.0
Net Cash Used in Investing Activities				0.0
Net Cash Used in Financing Activities				0.0
Net Decrease in Cash, Cash Equivalents and Restricted Cash				0.0
Cash, Cash Equivalents and Restricted Cash at Beginning of Period				0.0
Cash, Cash Equivalents and Restricted Cash at End of Period		$ 0.0		$ 0.0