Supplemental Cash Flow Information - Noncash Activity (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplemental Cash Flow Elements [Abstract]
Subvention receivable from GM	$ 189	$ 82	$ 21
Commercial loan funding payable to GM	$ 427	$ 362	$ 0