Commitments, Guarantees and Contingencies - Clean Water Act and Regulation of Waste Disposal (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Renewal cycle of waste water discharge permit	5 years
Regulation of Waste Disposal
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 125
Environmental liabilities former gas facilities	80
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