Derivative Financial Instruments - Summary of Proprietary Trading Activities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Derivative [Line Items]
Net mark-to-market gains (losses)	$ (8)	$ (26)	$ (39)
Change in fair value	8	1	1
Reclassification to realized at settlement	(10)	3	(3)
Operating Revenue [Member]
Derivative [Line Items]
Net mark-to-market gains (losses)	(6)	(30)	(37)
Change in fair value	8	(1)	(22)
Reclassification to realized at settlement	(14)	(29)	(15)
Interest Rate Swap [Member] | Operating Revenue [Member]
Derivative [Line Items]
Net mark-to-market gains (losses)	$ (2)	$ 4	$ (2)