Supplemental Information for the Consolidated Statements of Cash Flows - Noncash Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Supplemental Cash Flow Elements [Abstract]
Subvention receivable from GM	$ 642	$ 676	$ 727
Commercial loan funding payable to GM	$ 23	$ 74	$ 61