Commitments, Guarantees and Contingencies - Clean Water Act and Regulation of Waste Disposal (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015	Apr. 19, 2013 option
Loss Contingencies [Line Items]
Number of treatment options | option			8
Number of preferred treatment options | option			4
Renewal cycle of waste water discharge permit	5 years
Clean Water Act
Loss Contingencies [Line Items]
Annual percentage that fish impingement should be reduced to, per CWA	12.00%
TMDL limit development period	5 years
Regulation of Waste Disposal
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 137
Environmental liabilities former gas facilities	89
Bond closure and post closure period		45 years
Period of time to implement plan		12 years
Minimum | Clean Water Act
Loss Contingencies [Line Items]
Maximum capital investment required to install technology to meet TDS and Sulfate limits	150
Maximum | Clean Water Act
Loss Contingencies [Line Items]
Maximum capital investment required to install technology to meet TDS and Sulfate limits	$ 300