Quarterly Financial Data (unaudited) - Summary of Quarterly Financial Data (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
Quarterly Financial Information Disclosure [Abstract]
Net sales	$ 4,817.6	$ 4,825.0	$ 5,007.6	$ 5,160.1	$ 4,942.4	$ 4,891.6	$ 5,580.8	$ 5,733.7	$ 19,810.3	$ 21,148.5	$ 20,467.0
Gross profit	326.8	459.3	478.2	473.2	522.5	526.3	638.1	631.4
Consolidated net income	144.0	238.6	202.0	246.1	242.8	274.7	355.0	374.2	830.7	1,246.7	1,380.9
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
Basic net income per share attributable to Lear (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 attributable to Lear (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