Quarterly Financial Data (unaudited) - Summary of Quarterly Financial Data (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
Quarterly Financial Information Disclosure [Abstract]
Net sales	$ 5,243.2	$ 4,900.1	$ 2,444.5	$ 4,457.7	$ 4,817.6	$ 4,825.0	$ 5,007.6	$ 5,160.1	$ 17,045.5	$ 19,810.3	$ 21,148.5
Gross profit	459.3	442.8	(127.4)	334.2	326.8	459.3	478.2	473.2
Consolidated net income	222.7	197.1	(269.5)	83.6	144.0	238.6	202.0	246.1	233.9	830.7	1,246.7
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
Basic net income per share attributable to Lear (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 attributable to Lear (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