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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, 2017
Dec. 31, 2016
Goodwill [Roll Forward]    
Beginning balance $ 1,121.3 $ 1,053.8
Foreign currency translation and other 62.3 (16.0)
Ending balance 1,401.3 1,121.3
Other acquisitions    
Goodwill [Roll Forward]    
Acquisition 123.3 74.6
Beijing BAI Lear Automotive Systems Co., Ltd.    
Goodwill [Roll Forward]    
Acquisition   8.9
Shanghai Lear STEC Automotive Parts Co., Ltd. (China)    
Goodwill [Roll Forward]    
Acquisition 94.4  
Operating segments | Seating    
Goodwill [Roll Forward]    
Beginning balance 1,091.2 1,026.8
Foreign currency translation and other 59.9 (16.5)
Ending balance 1,274.4 1,091.2
Operating segments | Seating | Other acquisitions    
Goodwill [Roll Forward]    
Acquisition 123.3 72.0
Operating segments | Seating | Beijing BAI Lear Automotive Systems Co., Ltd.    
Goodwill [Roll Forward]    
Acquisition   8.9
Operating segments | Seating | Shanghai Lear STEC Automotive Parts Co., Ltd. (China)    
Goodwill [Roll Forward]    
Acquisition 0.0  
Operating segments | E-Systems    
Goodwill [Roll Forward]    
Beginning balance 30.1 27.0
Foreign currency translation and other 2.4 0.5
Ending balance 126.9 30.1
Operating segments | E-Systems | Other acquisitions    
Goodwill [Roll Forward]    
Acquisition 0.0 2.6
Operating segments | E-Systems | Beijing BAI Lear Automotive Systems Co., Ltd.    
Goodwill [Roll Forward]    
Acquisition   $ 0.0
Operating segments | E-Systems | Shanghai Lear STEC Automotive Parts Co., Ltd. (China)    
Goodwill [Roll Forward]    
Acquisition $ 94.4