Adoption of Revenue Recognition - Effect of Revenue Recognition on Condensed 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
Contract assets	253.1	268.1
Other current assets, net	123.2	129.1
Total Current Assets	1,070.5	1,016.7
Noncurrent Assets
Property, plant and equipment, net	357.8	359.0
Recoverable environmental remediation costs	225.8	231.1
Deferred income taxes	255.5	145.8
Goodwill	161.3	161.3
Intangible assets	78.7	85.5
Other noncurrent assets, net	260.5	259.3
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
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
Common stock	7.4	7.4
Other capital	509.4	503.1
Treasury stock at cost	(64.5)	(64.5)
Retained earnings (accumulated deficit)	15.4	(71.0)
Accumulated other comprehensive loss, net of income taxes	(247.6)	(272.6)
Total Stockholders’ Equity	220.1	102.4
Total Liabilities and Stockholders’ Equity	2,410.1	2,258.7
Effect of Adoption | ASU 2014-09
Current Assets
Cash and cash equivalents	0.0	0.0
Marketable securities	0.0
Accounts receivable	(55.9)
Contract assets	19.5
Other current assets, net	(10.5)
Total Current Assets	(46.9)
Noncurrent Assets
Property, plant and equipment, net	0.0
Recoverable environmental remediation costs	0.0
Deferred income taxes	(117.4)
Goodwill	0.0
Intangible assets	0.0
Other noncurrent assets, net	0.0
Total Noncurrent Assets	(117.4)
Total Assets	(164.3)
Current Liabilities
Current portion of long-term debt	0.0
Accounts payable	0.0
Reserves for environmental remediation costs	0.0
Contract liabilities	13.9
Other current liabilities	(132.3)
Total Current Liabilities	(118.4)
Total Noncurrent Liabilities	0.0
Total Liabilities	(118.4)
Stockholders’ Equity
Common stock	0.0
Other capital	0.0
Treasury stock at cost	0.0
Retained earnings (accumulated deficit)	(45.9)
Accumulated other comprehensive loss, net of income taxes	0.0
Total Stockholders’ Equity	(45.9)
Total Liabilities and Stockholders’ Equity	(164.3)
Amounts Excluding Effect of Adoption
Current Assets
Cash and cash equivalents	514.7	$ 535.0
Marketable securities	44.9
Accounts receivable	78.7
Contract assets	272.6
Other current assets, net	112.7
Total Current Assets	1,023.6
Noncurrent Assets
Property, plant and equipment, net	357.8
Recoverable environmental remediation costs	225.8
Deferred income taxes	138.1
Goodwill	161.3
Intangible assets	78.7
Other noncurrent assets, net	260.5
Total Noncurrent Assets	1,222.2
Total Assets	2,245.8
Current Liabilities
Current portion of long-term debt	30.4
Accounts payable	104.1
Reserves for environmental remediation costs	34.6
Contract liabilities	199.2
Other current liabilities	180.7
Total Current Liabilities	549.0
Total Noncurrent Liabilities	1,522.6
Total Liabilities	2,071.6
Stockholders’ Equity
Common stock	7.4
Other capital	509.4
Treasury stock at cost	(64.5)
Retained earnings (accumulated deficit)	(30.5)
Accumulated other comprehensive loss, net of income taxes	(247.6)
Total Stockholders’ Equity	174.2
Total Liabilities and Stockholders’ Equity	$ 2,245.8