Adoption of Revenue Recognition - Reclassification to Prior Period Consolidated Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating Activities
Net income	$ 14.0	$ 5.9
Adjustments to reconcile net income to net cash used in operating activities:
Depreciation and amortization	17.7	16.3
Amortization of debt discount and deferred financing costs	2.1	2.1
Stock-based compensation	1.5	6.8
Retirement benefits, net	3.3	7.7
Loss on disposal of long-lived assets	0.0	0.1
Changes in assets and liabilities, net of effects from acquisition in 2017:
Accounts receivable	(76.2)	(30.1)
Inventories		0.0
Contract assets	(20.4)	2.3
Other current assets, net	(17.4)	(6.2)
Recoverable environmental remediation costs	3.0	4.1
Other noncurrent assets	(3.9)	(3.4)
Accounts payable	29.8	1.1
Contract liabilities	(30.8)	(16.0)
Advance payments on contracts		0.0
Other current liabilities	(21.9)	7.7
Deferred income taxes	9.0	2.3
Reserves for environmental remediation costs	(3.0)	(4.2)
Other noncurrent liabilities and other	(0.3)	0.4
Net Cash Used in Operating Activities	(95.4)	(3.1)
Investing Activities
Net Cash Used in Investing Activities	(2.2)	(17.7)
Financing Activities
Net Cash Used in Financing Activities	(4.6)	(7.4)
Net Decrease in Cash and Cash Equivalents	(102.2)	(28.2)
Cash and Cash Equivalents at Beginning of Period	535.0	410.3
Cash and Cash Equivalents at End of Period	432.8	382.1
As Reported
Operating Activities
Net income		5.9
Adjustments to reconcile net income to net cash used in operating activities:
Depreciation and amortization		16.3
Amortization of debt discount and deferred financing costs		2.1
Stock-based compensation		6.8
Retirement benefits, net		7.7
Loss on disposal of long-lived assets		0.1
Changes in assets and liabilities, net of effects from acquisition in 2017:
Accounts receivable		(54.3)
Inventories		24.7
Contract assets		0.0
Other current assets, net		(4.4)
Recoverable environmental remediation costs		4.1
Other noncurrent assets		(3.4)
Accounts payable		1.1
Contract liabilities		0.0
Advance payments on contracts		(13.2)
Other current liabilities		4.9
Deferred income taxes		2.3
Reserves for environmental remediation costs		(4.2)
Other noncurrent liabilities and other		0.4
Net Cash Used in Operating Activities		(3.1)
Investing Activities
Net Cash Used in Investing Activities		(17.7)
Financing Activities
Net Cash Used in Financing Activities		(7.4)
Net Decrease in Cash and Cash Equivalents		(28.2)
Cash and Cash Equivalents at Beginning of Period	535.0	410.3
Cash and Cash Equivalents at End of Period		382.1
ASU 2014-09 | Reclassifications due to Adoption
Operating Activities
Net income		0.0
Adjustments to reconcile net income to net cash used in operating activities:
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 disposal of long-lived assets		0.0
Changes in assets and liabilities, net of effects from acquisition in 2017:
Accounts receivable		24.2
Inventories		(24.7)
Contract assets		2.3
Other current assets, net		(1.8)
Recoverable environmental remediation costs		0.0
Other noncurrent assets		0.0
Accounts payable		0.0
Contract liabilities		(16.0)
Advance payments on contracts		13.2
Other current liabilities		2.8
Deferred income taxes		0.0
Reserves for environmental remediation costs		0.0
Other noncurrent liabilities and other		0.0
Net Cash Used in Operating Activities		0.0
Investing Activities
Net Cash Used in Investing Activities		0.0
Financing Activities
Net Cash Used in Financing Activities		0.0
Net Decrease in Cash and Cash Equivalents		0.0
Cash and Cash Equivalents at Beginning of Period	$ 0.0	0.0
Cash and Cash Equivalents at End of Period		$ 0.0