Derivative Financial Instruments - Summary of Proprietary Trading Activities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative [Line Items]
Change in fair value	$ 1	$ 1	$ 1
Reclassification to realized at settlement	3	(3)	0
Net mark-to-market gains (losses)	(26)	(39)	96
Operating Revenue [Member]
Derivative [Line Items]
Change in fair value	(1)	(22)	(13)
Reclassification to realized at settlement	(29)	(15)	108
Net mark-to-market gains (losses)	(30)	(37)	95
Interest Rate Swap [Member] | Operating Revenue [Member]
Derivative [Line Items]
Net mark-to-market gains (losses)	$ 4	$ (2)	$ 1