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, 2019	Sep. 28, 2019	Jun. 29, 2019	Mar. 30, 2019	Dec. 31, 2018	Sep. 29, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Net income attributable to Lear	$ 126.0	$ 215.9	$ 182.8	$ 228.9	$ 212.2	$ 252.5	$ 331.4	$ 353.7	$ 753.6	$ 1,149.8	$ 1,313.4
Redeemable noncontrolling interest adjustment									35.9	(10.4)	(25.5)
Net income available to Lear common stockholders									$ 789.5	$ 1,139.4	$ 1,287.9
Average common shares outstanding (in shares)									61,697,192	65,672,164	68,542,363
Dilutive effect of common stock equivalents (in shares)									226,336	489,652	735,618
Average diluted shares outstanding (in shares)									61,923,528	66,161,816	69,277,981
Basic net income per share available to Lear common stockholders (in dollars per share)	$ 2.51	$ 3.59	$ 2.92	$ 3.75	$ 3.42	$ 3.83	$ 4.86	$ 5.19	$ 12.80	$ 17.35	$ 18.79
Diluted net income per share available to Lear common stockholders (in dollars per share)	$ 2.50	$ 3.58	$ 2.92	$ 3.73	$ 3.39	$ 3.80	$ 4.83	$ 5.16	$ 12.75	$ 17.22	$ 18.59