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Accounting Policies (Accounting Pronouncements) (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unfavorable impact on pre-tax operating results		$ 121.3	$ 531.5	$ 86.9	$ 705.2
Accounting Standards Update 2017-07 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Projected impact of new accounting principle on prior period operating income							$ 70.6
Forecast | Accounting Standards Update 2017-07 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unfavorable impact on pre-tax operating results	$ 5.0
High Performance Materials & Components | Forecast
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Customer agreements as a percentage of annualized revenue						5.00%

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(h)(1)(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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