Adoption of Revenue Recognition - Reclassification to Prior Period Consolidated Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operating Activities
Net income	$ 34.8	$ 24.3	$ 48.8	$ 30.2
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	17.7	19.1	35.4	35.4
Amortization of debt discount and deferred financing costs			4.4	4.2
Stock-based compensation	5.2	3.4	6.7	10.2
Retirement benefits, net			6.7	15.4
Other, net			(0.1)	0.3
Changes in assets and liabilities, net of effects from acquisition in 2017:
Accounts receivable			(40.7)	(50.3)
Inventories				0.0
Contract assets			(7.5)	(9.7)
Other current assets, net			16.3	12.2
Recoverable environmental remediation costs			5.3	10.1
Other noncurrent assets			(1.5)	(31.5)
Accounts payable			(9.7)	12.6
Contract liabilities			(58.1)	(43.0)
Advance payments on contracts				0.0
Other current liabilities			21.8	13.6
Deferred income taxes			2.3	15.9
Reserves for environmental remediation costs			(7.7)	(10.8)
Other noncurrent liabilities and other			4.0	22.9
Net Cash Provided by Operating Activities			24.5	37.7
Investing Activities
Net Cash Used in Investing Activities			(34.9)	(21.1)
Financing Activities
Net Cash Used in Financing Activities			(9.9)	(13.3)
Net (Decrease) Increase in Cash and Cash Equivalents			(20.3)	3.3
Cash and Cash Equivalents at Beginning of Period			535.0	410.3
Cash and Cash Equivalents at End of Period	$ 514.7	413.6	514.7	413.6
As Reported
Operating Activities
Net income				30.2
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization				35.4
Amortization of debt discount and deferred financing costs				4.2
Stock-based compensation				10.2
Retirement benefits, net				15.4
Other, net				0.3
Changes in assets and liabilities, net of effects from acquisition in 2017:
Accounts receivable				(87.8)
Inventories				20.7
Contract assets				0.0
Other current assets, net				19.3
Recoverable environmental remediation costs				10.1
Other noncurrent assets				(31.5)
Accounts payable				12.6
Contract liabilities				0.0
Advance payments on contracts				(49.0)
Other current liabilities				19.6
Deferred income taxes				15.9
Reserves for environmental remediation costs				(10.8)
Other noncurrent liabilities and other				22.9
Net Cash Provided by Operating Activities				37.7
Investing Activities
Net Cash Used in Investing Activities				(21.1)
Financing Activities
Net Cash Used in Financing Activities				(13.3)
Net (Decrease) Increase in Cash and Cash Equivalents				3.3
Cash and Cash Equivalents at Beginning of Period			535.0	410.3
Cash and Cash Equivalents at End of Period		413.6		413.6
ASU 2014-09 | Reclassifications due to Adoption
Operating Activities
Net income				0.0
Adjustments to reconcile net income to net cash provided by 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
Other, net				0.0
Changes in assets and liabilities, net of effects from acquisition in 2017:
Accounts receivable				37.5
Inventories				(20.7)
Contract assets				(9.7)
Other current assets, net				(7.1)
Recoverable environmental remediation costs				0.0
Other noncurrent assets				0.0
Accounts payable				0.0
Contract liabilities				(43.0)
Advance payments on contracts				49.0
Other current liabilities				(6.0)
Deferred income taxes				0.0
Reserves for environmental remediation costs				0.0
Other noncurrent liabilities and other				0.0
Net Cash Provided by 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) Increase 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		$ 0.0