Summary of Significant Accounting Policies - Schedule of Deferred Revenue Activity (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Deferred revenue on automotive sales with and without resale value guarantee— beginning of period	$ 1,472	$ 883
Additions	724	880
Net changes in liability for pre-existing contracts	56	9
Revenue recognized	(326)	(300)
Deferred revenue on automotive sales with and without resale value guarantee— end of period	$ 1,926	$ 1,472