Adoption of Revenue Recognition - Reclassification to December 31, 2017 Consolidated Balance Sheet (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Current Assets
Cash and cash equivalents	$ 514.7	$ 535.0	$ 413.6	$ 410.3
Marketable securities	44.9	20.0
Accounts receivable	134.6	64.5
Inventories		0.0
Contract assets	253.1	268.1
Other current assets, net	123.2	129.1
Total Current Assets	1,070.5	1,016.7
Total Noncurrent Assets	1,339.6	1,242.0
Total Assets	2,410.1	2,258.7
Current Liabilities
Current portion of long-term debt	30.4	25.0
Accounts payable	104.1	100.9
Reserves for environmental remediation costs	34.6	35.2
Contract liabilities	185.3	276.8
Advance payments on contracts		0.0
Other current liabilities	313.0	156.9
Total Current Liabilities	667.4	594.8
Total Noncurrent Liabilities	1,522.6	1,561.5
Total Liabilities	2,190.0	2,156.3
Stockholders’ Equity
Total Stockholders’ Equity	220.1	102.4
Total Liabilities and Stockholders’ Equity	$ 2,410.1	2,258.7
As Reported
Current Assets
Cash and cash equivalents		535.0	413.6	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