Employee Benefit Plans - Net Periodic Benefit Costs Not Yet Recognized (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Pension Plans [Member] | Net Regulatory Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	$ 0	$ 0
Net loss	88	76
Total	88	76
Pension Plans [Member] | Accumulated OCI [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	(4)	(6)
Net loss	286	307
Total	282	301
Pension Plans [Member] | Total Net Regulatory Assets And AOCI [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	(4)	(6)
Net loss	374	383
Total	370	377
Welfare Plans [Member] | Net Regulatory Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	(15)	(18)
Net loss	45	57
Total	30	39
Welfare Plans [Member] | Accumulated OCI [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	0	0
Net loss	36	36
Total	36	36
Welfare Plans [Member] | Total Net Regulatory Assets And AOCI [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	(15)	(18)
Net loss	81	93
Total	$ 66	$ 75