Adoption of Revenue Recognition - Effect of Revenue Recognition on 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
Insurance proceeds	(1.9)	0.0
Changes in assets and liabilities:
Accounts receivable	(76.2)	(30.1)
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)
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
Insurance proceeds	1.9	0.0
Capital expenditures	(4.1)	(2.7)
Net Cash Used in Investing Activities	(2.2)	(17.7)
Financing Activities
Debt repayments	(5.1)	(5.0)
Repurchase of shares for withholding taxes and option costs under employee equity plans	(1.6)	(4.7)
Proceeds from shares issued under equity plans	2.1	2.3
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
Amounts Excluding Effect of Adoption
Operating Activities
Net income	8.1
Adjustments to reconcile net income to net cash used in operating activities:
Depreciation and amortization	17.7
Amortization of debt discount and deferred financing costs	2.1
Stock-based compensation	1.5
Retirement benefits, net	3.3
Insurance proceeds	(1.9)
Changes in assets and liabilities:
Accounts receivable	(24.5)
Contract assets	(34.0)
Other current assets, net	(16.6)
Recoverable environmental remediation costs	3.0
Other noncurrent assets	(3.9)
Accounts payable	29.8
Contract liabilities	(63.6)
Other current liabilities	(20.1)
Deferred income taxes	7.0
Reserves for environmental remediation costs	(3.0)
Other noncurrent liabilities and other	(0.3)
Net Cash Used in Operating Activities	(95.4)
Investing Activities
Insurance proceeds	1.9
Capital expenditures	(4.1)
Net Cash Used in Investing Activities	(2.2)
Financing Activities
Debt repayments	(5.1)
Repurchase of shares for withholding taxes and option costs under employee equity plans	(1.6)
Proceeds from shares issued under equity plans	2.1
Net Cash Used in Financing Activities	(4.6)
Net Decrease in Cash and Cash Equivalents	(102.2)
Cash and Cash Equivalents at Beginning of Period	535.0
Cash and Cash Equivalents at End of Period	432.8
Effect of Adoption | ASU 2014-09
Operating Activities
Net income	(5.9)
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
Insurance proceeds	0.0
Changes in assets and liabilities:
Accounts receivable	51.7
Contract assets	(13.6)
Other current assets, net	0.8
Recoverable environmental remediation costs	0.0
Other noncurrent assets	0.0
Accounts payable	0.0
Contract liabilities	(32.8)
Other current liabilities	1.8
Deferred income taxes	(2.0)
Reserves for environmental remediation costs	0.0
Other noncurrent liabilities and other	0.0
Net Cash Used in Operating Activities	0.0
Investing Activities
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 in Cash and Cash Equivalents	0.0
Cash and Cash Equivalents at Beginning of Period	0.0
Cash and Cash Equivalents at End of Period	$ 0.0