Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Activities
Net income	$ 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, net of effects from acquisition in 2019:
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 for 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
Capital expenditures	(54.6)	(42.9)	(43.2)
Purchases of marketable securities	(31.7)	0.0	(47.7)
Sale of marketable securities	25.0	0.0	68.1
Other	0.3	1.1	1.9
Net Cash Used in Investing Activities	(61.0)	(41.8)	(20.9)
Financing Activities
Debt issuance costs	0.0	0.0	(3.3)
Debt principal payments	(21.7)	(20.8)	(25.3)
Proceeds from shares issued under equity plans	7.5	5.2	5.4
Repurchase of shares for withholding taxes and option costs under employee equity plans	(20.0)	(8.5)	(3.3)
Purchase of treasury stock	(51.7)	0.0	0.0
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	535.0
Cash, Cash Equivalents and Restricted Cash at End of Year	1,152.5	935.6	740.3
Supplemental disclosures of cash flow information
Cash paid for interest	19.7	26.3	25.3
Cash paid for income taxes	56.0	63.8	24.0
Cash refund for income taxes	13.3	2.0	5.2
Contribution of treasury stock to retirement benefit plans	$ 0.0	$ 0.0	$ 95.0