Employee Benefit Plans - Estimated Amortization Out of Regulatory Assets or Accumulated OCI (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($)
Pension Plans [Member] | Net Regulatory Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service credit	$ 0
Amortization of net loss	7
Pension Plans [Member] | Accumulated OCI [Member]
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service credit	(2)
Amortization of net loss	17
Pension Plans [Member] | Total Net Regulatory Assets And AOCI [Member]
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service credit	(2)
Amortization of net loss	24
Welfare Plans [Member] | Net Regulatory Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service credit	(3)
Amortization of net loss	2
Welfare Plans [Member] | Accumulated OCI [Member]
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service credit	0
Amortization of net loss	3
Welfare Plans [Member] | Total Net Regulatory Assets And AOCI [Member]
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service credit	(3)
Amortization of net loss	$ 5