Adoption of Revenue Recognition - Reclassification to December 31, 2017 Consolidated Balance Sheet (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Current Assets
Cash and cash equivalents	$ 432.8	$ 535.0	$ 382.1	$ 410.3
Marketable securities	20.0	20.0
Accounts receivable	170.1	64.5
Inventories		0.0
Contract assets	266.0	268.1
Other current assets, net	156.7	129.1
Total Current Assets	1,045.6	1,016.7
Total Noncurrent Assets	1,340.3	1,242.0
Total Assets	2,385.9	2,258.7
Current Liabilities
Current portion of long-term debt	27.7	25.0
Accounts payable	133.4	100.9
Reserves for environmental remediation costs	36.4	35.2
Contract liabilities	212.6	276.8
Advance payments on contracts		0.0
Other current liabilities	267.6	156.9
Total Current Liabilities	677.7	594.8
Total Noncurrent Liabilities	1,539.0	1,561.5
Total Liabilities	2,216.7	2,156.3
Stockholders’ Equity
Total Stockholders’ Equity	169.2	102.4
Total Liabilities and Stockholders’ Equity	$ 2,385.9	2,258.7
As Reported
Current Assets
Cash and cash equivalents		535.0	382.1	410.3
Marketable securities		20.0
Accounts receivable		215.5
Inventories		136.4
Contract assets		0.0
Other current assets, net		109.8
Total Current Assets		1,016.7
Total Noncurrent Assets		1,242.0
Total Assets		2,258.7
Current Liabilities
Current portion of long-term debt		25.0
Accounts payable		100.9
Reserves for environmental remediation costs		35.2
Contract liabilities		0.0
Advance payments on contracts		237.8
Other current liabilities		195.9
Total Current Liabilities		594.8
Total Noncurrent Liabilities		1,561.5
Total Liabilities		2,156.3
Stockholders’ Equity
Total Stockholders’ Equity		102.4
Total Liabilities and Stockholders’ Equity		2,258.7
ASU 2014-09 | Reclassifications due to Adoption
Current Assets
Cash and cash equivalents		0.0	$ 0.0	$ 0.0
Marketable securities		0.0
Accounts receivable		(151.0)
Inventories		(136.4)
Contract assets		268.1
Other current assets, net		19.3
Total Current Assets		0.0
Total Noncurrent Assets		0.0
Total Assets		0.0
Current Liabilities
Current portion of long-term debt		0.0
Accounts payable		0.0
Reserves for environmental remediation costs		0.0
Contract liabilities		276.8
Advance payments on contracts		(237.8)
Other current liabilities		(39.0)
Total Current Liabilities		0.0
Total Noncurrent Liabilities		0.0
Total Liabilities		0.0
Stockholders’ Equity
Total Stockholders’ Equity		0.0
Total Liabilities and Stockholders’ Equity		$ 0.0