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Summary of Significant Accounting Policies - Summary of Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Beginning balance	$ 1,401.3	$ 1,121.3
Foreign currency translation and other	(18.4)	62.3
Ending balance	1,405.3	1,401.3
Other acquisitions
Goodwill [Roll Forward]
Acquisitions		123.3
Shanghai Lear STEC Automotive Parts Co., Ltd. (China)
Goodwill [Roll Forward]
Acquisitions		94.4
Lear FAWSN
Goodwill [Roll Forward]
Acquisitions	22.4
Seating
Goodwill [Roll Forward]
Beginning balance	1,274.4	1,091.2
Foreign currency translation and other	(30.1)	59.9
Ending balance	1,244.3	1,274.4
Seating | Other acquisitions
Goodwill [Roll Forward]
Acquisitions		123.3
Seating | Shanghai Lear STEC Automotive Parts Co., Ltd. (China)
Goodwill [Roll Forward]
Acquisitions		0.0
Seating | Lear FAWSN
Goodwill [Roll Forward]
Acquisitions	0.0
E-Systems
Goodwill [Roll Forward]
Beginning balance	126.9	30.1
Foreign currency translation and other	11.7	2.4
Ending balance	161.0	126.9
E-Systems | Other acquisitions
Goodwill [Roll Forward]
Acquisitions		0.0
E-Systems | Shanghai Lear STEC Automotive Parts Co., Ltd. (China)
Goodwill [Roll Forward]
Acquisitions		$ 94.4
E-Systems | Lear FAWSN
Goodwill [Roll Forward]
Acquisitions	$ 22.4

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- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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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