Investments in Affiliates and Other Related Party Transactions - Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Jan. 31, 2018	Dec. 31, 2017	Sep. 30, 2017
Business Acquisition [Line Items]
Goodwill	$ 1,614.3	$ 1,405.3		$ 1,401.3
Lear FAWSN
Business Acquisition [Line Items]
Property, plant and equipment			$ 11.0
Other assets and liabilities assumed, net			5.7
Goodwill			22.4
Intangible assets			7.5
Purchase price allocation			$ 46.6
Lear STEC
Business Acquisition [Line Items]
Property, plant and equipment					$ 16.2
Other assets and liabilities assumed, net					42.7
Goodwill					94.1
Intangible assets					66.0
Purchase price allocation					$ 219.0