Employee Benefit Plans (Reconciliation of Assets Measured at Fair Value on a Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 111	$ 13	$ 85
Purchases	84	30	36
Sales	(58)	(6)	(2)
Total gains (losses) and other, net	(7)	(42)	8
Ending balance	155	111	13
Master Trust [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	546	352
Combination of trust assets		288
Purchases	17	25
Sales	(164)	(152)
Total gains (losses) and other, net	37	33
Ending balance	$ 436	$ 546