Summary of Significant Accounting Policies - Summary of Information Used to Compute Basic and Diluted Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Oct. 03, 2020	Jul. 04, 2020	Apr. 04, 2020	Dec. 31, 2019	Sep. 28, 2019	Jun. 29, 2019	Mar. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Net income (loss) attributable to Lear	$ 201.6	$ 174.4	$ (293.9)	$ 76.4	$ 126.0	$ 215.9	$ 182.8	$ 228.9	$ 158.5	$ 753.6	$ 1,149.8
Redeemable noncontrolling interest adjustment									0.0	35.9	(10.4)
Net income available to Lear common stockholders									$ 158.5	$ 789.5	$ 1,139.4
Average common shares outstanding (in shares)									60,254,380	61,697,192	65,672,164
Dilutive effect of common stock equivalents (in shares)									172,582	226,336	489,652
Average diluted shares outstanding (in shares)									60,426,962	61,923,528	66,161,816
Basic net income per share available to Lear common stockholders (in dollars per share)	$ 3.35	$ 2.90	$ (4.89)	$ 1.26	$ 2.51	$ 3.59	$ 2.92	$ 3.75	$ 2.63	$ 12.80	$ 17.35
Diluted net income per share available to Lear common stockholders (in dollars per share)	$ 3.33	$ 2.89	$ (4.89)	$ 1.26	$ 2.50	$ 3.58	$ 2.92	$ 3.73	$ 2.62	$ 12.75	$ 17.22