Adoption of Revenue Recognition - Effect of Revenue Recognition on Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Mar. 31, 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
Insurance proceeds				(1.9)	0.0
Other, net				(0.1)	0.3
Changes in assets and liabilities:
Accounts receivable				(40.7)	(50.3)
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)
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
Purchases of marketable securities				(44.7)	0.0
Sales of marketable securities				20.1	0.0
Insurance proceeds				1.9	0.0
Capital expenditures				(12.2)	(6.1)
Net Cash Used in Investing Activities				(34.9)	(21.1)
Financing Activities
Debt repayments				(10.3)	(10.0)
Repurchase of shares for withholding taxes and option costs under employee equity plans				(2.1)	(5.7)
Proceeds from shares issued under equity plans				2.5	2.4
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		535.0	410.3
Cash and Cash Equivalents at End of Period	514.7		$ 413.6	514.7	$ 413.6
Amounts Excluding Effect of Adoption
Operating Activities
Net income	32.4			40.5
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	17.7			35.4
Amortization of debt discount and deferred financing costs				4.4
Stock-based compensation				6.7
Retirement benefits, net				6.7
Insurance proceeds				(1.9)
Other, net				(0.1)
Changes in assets and liabilities:
Accounts receivable				(14.2)
Contract assets				(4.5)
Other current assets, net				16.7
Recoverable environmental remediation costs				5.3
Other noncurrent assets				(1.5)
Accounts payable				(9.7)
Contract liabilities				(77.6)
Other current liabilities				22.8
Deferred income taxes				(0.8)
Reserves for environmental remediation costs				(7.7)
Other noncurrent liabilities and other				4.0
Net Cash Provided by Operating Activities				24.5
Investing Activities
Purchases of marketable securities		(44.7)
Sales of marketable securities		20.1
Insurance proceeds				1.9
Capital expenditures				(12.2)
Net Cash Used in Investing Activities				(34.9)
Financing Activities
Debt repayments				(10.3)
Repurchase of shares for withholding taxes and option costs under employee equity plans				(2.1)
Proceeds from shares issued under equity plans				2.5
Net Cash Used in Financing Activities				(9.9)
Net (Decrease) Increase in Cash and Cash Equivalents				(20.3)
Cash and Cash Equivalents at Beginning of Period		535.0		535.0
Cash and Cash Equivalents at End of Period	514.7			514.7
Effect of Adoption | ASU 2014-09
Operating Activities
Net income	(2.4)			(8.3)
Adjustments to reconcile net income to net cash provided by operating activities:
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
Insurance proceeds				0.0
Other, net				0.0
Changes in assets and liabilities:
Accounts receivable				26.5
Contract assets				3.0
Other current assets, net				0.4
Recoverable environmental remediation costs				0.0
Other noncurrent assets				0.0
Accounts payable				0.0
Contract liabilities				(19.5)
Other current liabilities				1.0
Deferred income taxes				(3.1)
Reserves for environmental remediation costs				0.0
Other noncurrent liabilities and other				0.0
Net Cash Provided by Operating Activities				0.0
Investing Activities
Purchases of marketable securities		0.0
Sales of marketable securities		0.0
Insurance proceeds				0.0
Capital expenditures				0.0
Net Cash Used in Investing Activities				0.0
Financing Activities
Debt repayments				0.0
Repurchase of shares for withholding taxes and option costs under employee equity plans				0.0
Proceeds from shares issued under equity plans				0.0
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