Summary of Significant Accounting Policies - Summary of Information Used to Compute Basic and Diluted Net Income Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 29, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jul. 01, 2017	Apr. 01, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Net income attributable to Lear	$ 212.2	$ 252.5	$ 331.4	$ 353.7	$ 400.5	$ 295.2	$ 311.9	$ 305.8	$ 1,149.8	$ 1,313.4	$ 975.1
Less: Redeemable noncontrolling interest adjustment									(10.4)	(25.5)	0.0
Net income available to Lear common stockholders									$ 1,139.4	$ 1,287.9	$ 975.1
Average common shares outstanding (in shares)									65,672,164	68,542,363	72,345,436
Dilutive effect of common stock equivalents (in shares)									489,652	735,618	779,513
Average diluted shares outstanding (in shares)									66,161,816	69,277,981	73,124,949
Basic net income per share available to Lear common stockholders (in dollars per share)	$ 3.42	$ 3.83	$ 4.86	$ 5.19	$ 5.89	$ 4.00	$ 4.53	$ 4.39	$ 17.35	$ 18.79	$ 13.48
Diluted net income per share available to Lear common stockholders (in dollars per share)	$ 3.39	$ 3.80	$ 4.83	$ 5.16	$ 5.80	$ 3.96	$ 4.49	$ 4.35	$ 17.22	$ 18.59	$ 13.33