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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 |

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