Supplemental Cash Flow Information - Noncash Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplemental Cash Flow Elements [Abstract]
Subvention receivable from GM	$ 727	$ 306	$ 347
Commercial loan funding payable to GM	$ 61	$ 90	$ 320