Fair Value Measurements (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Interest Receivable		$ 2
Marketable securities, gain (loss)		$ 14	$ 28	$ 24
Asset-backed Securities [Member] | Standard & Poor's, A Rating [Member] | Fitch, A Rating [Member] | Moody's, A2 Rating [Member]
Percent of investments with credit quality ratings less than single A and A2		1.00%