Supplemental Cash Flow Information - Noncash Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Supplemental Cash Flow Elements [Abstract]
Subvention receivable from GM	$ 383	$ 189	$ 82
Commercial loan funding payable to GM	$ 351	$ 427	$ 362