Adoption of Revenue Recognition Guidance - Effect of Revenue Recognition on Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Activities
Net income	$ 35.4	$ 31.7	$ 39.2	$ 31.4	$ 25.3	$ 32.9	$ 44.1	$ 38.7	$ 137.7	$ 141.0	$ 137.3
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization									65.3	74.5	72.3
Amortization of debt discount and deferred financing costs									9.9	9.4	8.9
Stock-based compensation									31.4	27.3	20.5
Retirement benefits, net									18.1	21.6	15.9
Other, net									(0.4)	(0.6)	(2.2)
Changes in assets and liabilities:
Accounts receivable, net									36.9	28.8	(47.3)
Contract assets									(64.5)	11.0	10.5
Other current assets, net									9.2	(27.9)	21.5
Recoverable environmental remediation costs									7.1	16.3	(20.0)
Other noncurrent assets									0.6	(6.3)	5.8
Accounts payable									(36.8)	20.3	(39.4)
Contract liabilities									144.9	(10.3)	29.2
Other current liabilities									(4.4)	(72.5)	40.9
Deferred income taxes									37.9	(5.7)	4.7
Reserves for environmental remediation costs									(8.6)	(18.7)	(13.5)
Other noncurrent liabilities and other									(20.5)	53.0	7.6
Net Cash Provided by Operating Activities									363.8	261.2	252.7
Investing Activities
Net Cash Used in Investing Activities									(61.0)	(41.8)	(20.9)
Financing Activities
Net Cash Used in Financing Activities									(85.9)	(24.1)	(26.5)
Net Increase in Cash, Cash Equivalents and Restricted Cash									216.9	195.3	205.3
Cash, Cash Equivalents and Restricted Cash at Beginning of Year				$ 935.6				740.3	935.6	740.3	535.0
Cash, Cash Equivalents and Restricted Cash at End of Year	$ 1,152.5				$ 935.6				$ 1,152.5	935.6	740.3
Effect of Adoption | ASU 2014-09
Operating Activities
Net income											(9.6)
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:
Accounts receivable, net											57.1
Contract assets											1.2
Other current assets, net											(8.1)
Recoverable environmental remediation costs											0.0
Other noncurrent assets											0.0
Accounts payable											0.0
Contract liabilities											(42.2)
Other current liabilities											(31.8)
Deferred income taxes											33.4
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 Increase in Cash, Cash Equivalents and Restricted Cash											0.0
Cash, Cash Equivalents and Restricted Cash at Beginning of Year								0.0		0.0	0.0
Cash, Cash Equivalents and Restricted Cash at End of Year											0.0
Amounts Excluding Effect of Adoption
Operating Activities
Net income											127.7
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization											72.3
Amortization of debt discount and deferred financing costs											8.9
Stock-based compensation											20.5
Retirement benefits, net											15.9
Other, net											(2.2)
Changes in assets and liabilities:
Accounts receivable, net											9.8
Contract assets											11.7
Other current assets, net											13.4
Recoverable environmental remediation costs											(20.0)
Other noncurrent assets											5.8
Accounts payable											(39.4)
Contract liabilities											(13.0)
Other current liabilities											9.1
Deferred income taxes											38.1
Reserves for environmental remediation costs											(13.5)
Other noncurrent liabilities and other											7.6
Net Cash Provided by Operating Activities											252.7
Investing Activities
Net Cash Used in Investing Activities											(20.9)
Financing Activities
Net Cash Used in Financing Activities											(26.5)
Net Increase in Cash, Cash Equivalents and Restricted Cash											205.3
Cash, Cash Equivalents and Restricted Cash at Beginning of Year								$ 740.3		$ 740.3	535.0
Cash, Cash Equivalents and Restricted Cash at End of Year											$ 740.3