Goodwill and Intangible Assets Goodwill and Other Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Beginning balance	$ 154.0	$ 154.4
Goodwill, Translation and Purchase Accounting Adjustments	27.5	(0.4)
Ending balance	1,654.3	154.0
Driveline [Member]
Goodwill [Roll Forward]
Beginning balance	130.1	130.5
Goodwill, Translation and Purchase Accounting Adjustments	0.6	(0.4)
Ending balance	211.1	130.1
Metal Forming [Member]
Goodwill [Roll Forward]
Beginning balance	23.9	23.9
Goodwill, Translation and Purchase Accounting Adjustments	19.7	0.0
Ending balance	558.9	23.9
Powertrain [Member]
Goodwill [Roll Forward]
Beginning balance	0.0	0.0
Goodwill, Translation and Purchase Accounting Adjustments	7.2	0.0
Ending balance	478.8	0.0
Casting [Member]
Goodwill [Roll Forward]
Beginning balance	0.0	0.0
Goodwill, Translation and Purchase Accounting Adjustments	0.0	0.0
Ending balance	405.5	$ 0.0
Metaldyne Performance Group, Inc. [Member]
Goodwill [Roll Forward]
Goodwill, Acquired During Period	1,392.4
Metaldyne Performance Group, Inc. [Member] \| Driveline [Member]
Goodwill [Roll Forward]
Goodwill, Acquired During Period	0.0
Metaldyne Performance Group, Inc. [Member] \| Metal Forming [Member]
Goodwill [Roll Forward]
Goodwill, Acquired During Period	515.3
Metaldyne Performance Group, Inc. [Member] \| Powertrain [Member]
Goodwill [Roll Forward]
Goodwill, Acquired During Period	471.6
Metaldyne Performance Group, Inc. [Member] \| Casting [Member]
Goodwill [Roll Forward]
Goodwill, Acquired During Period	405.5
USM Mexico Manufacturing LLC [Member]
Goodwill [Roll Forward]
Goodwill, Acquired During Period	80.4
USM Mexico Manufacturing LLC [Member] \| Driveline [Member]
Goodwill [Roll Forward]
Goodwill, Acquired During Period	80.4
USM Mexico Manufacturing LLC [Member] \| Metal Forming [Member]
Goodwill [Roll Forward]
Goodwill, Acquired During Period	0.0
USM Mexico Manufacturing LLC [Member] \| Powertrain [Member]
Goodwill [Roll Forward]
Goodwill, Acquired During Period	0.0
USM Mexico Manufacturing LLC [Member] \| Casting [Member]
Goodwill [Roll Forward]
Goodwill, Acquired During Period	$ 0.0

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

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Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

+ References

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

Amount of increase (decrease) from foreign currency translation adjustments and purchase accounting adjustments of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 16
-URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e961-128460

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

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