Revision of Previously Issued Financial Statements - Condensed Consolidated Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Reclassification [Line Items]
Net income	$ 27.8	$ 18.1
Depreciation and amortization	14.4	15.2
Amortization of debt discount and deferred financing costs	0.4	1.5
Stock-based compensation	(0.9)	2.8
Retirement benefits, net	(6.6)	(2.5)
Loss on debt	0.0	9.1
Other, net	0.5	(0.3)
Accounts receivable	(82.8)	(130.7)
Contract assets	(17.8)	(11.9)
Other current assets	5.9	14.1
Recoverable environmental remediation costs	5.5	3.9
Other noncurrent assets	6.8	2.3
Accounts payable	(19.0)	(27.4)
Contract liabilities	0.2	29.9
Other current liabilities	45.3	14.6
Deferred income taxes	(45.8)	(3.0)
Reserves for environmental remediation costs	(2.7)	(5.3)
Other noncurrent liabilities and other	(6.2)	0.2
Net Cash Used in Operating Activities	(75.0)	(69.4)
Net Cash Used in Investing Activities	(2.2)	(5.7)
Net Cash Used in Financing Activities	(12.3)	(570.7)
Net Decrease in Cash, Cash Equivalents and Restricted Cash	(89.5)	(645.8)
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	$ 613.9	506.7
Previously Reported
Reclassification [Line Items]
Net income		18.1
Depreciation and amortization		15.2
Amortization of debt discount and deferred financing costs		1.5
Stock-based compensation		2.8
Retirement benefits, net		(2.5)
Loss on debt		9.1
Other, net		(0.3)
Accounts receivable		(130.7)
Contract assets		(11.9)
Other current assets		11.1
Recoverable environmental remediation costs		3.9
Other noncurrent assets		2.3
Accounts payable		(27.4)
Contract liabilities		29.9
Other current liabilities		14.6
Deferred income taxes		0.0
Reserves for environmental remediation costs		(5.3)
Other noncurrent liabilities and other		0.2
Net Cash Used in Operating Activities		(69.4)
Net Cash Used in Investing Activities		(5.7)
Net Cash Used in Financing Activities		(570.7)
Net Decrease in Cash, Cash Equivalents and Restricted Cash		(645.8)
Cash, Cash Equivalents and Restricted Cash at Beginning of Period		1,152.5
Cash, Cash Equivalents and Restricted Cash at End of Period		506.7
Adjustments
Reclassification [Line Items]
Net income		0.0
Depreciation and amortization		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		3.0
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		(3.0)
Reserves for environmental remediation costs		0.0
Other noncurrent liabilities and other
Net Cash Used in 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