Income Taxes Unrecognized Tax Benefit (Details) - USD ($)		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Income Tax Examination [Line Items]
Unrecognized Tax Benefits, Interest on Income Taxes Accrued		$ 0	$ 0		$ 0		$ (3,000,000)
Interest Expense (Income) related to unrecognized tax benefits		$ 0	$ 0		$ 3,000,000
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%	21.00%	[1]	35.00%	[1]	35.00%
Unrecognized Tax Benefits, Income Tax Penalties Accrued		$ 0	$ 0		$ 0
Unrecognized Tax Benefits that Would Impact Effective Tax Rate		$ 35,000,000	$ 28,000,000
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent		4.90%	5.00%	[1]	4.10%	[1]
Effective Income Tax Rate Reconciliation, Tax Credit, Percent		(9.40%)	(5.20%)	[1]	(4.70%)	[1]
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items, Percent	[2]	5.80%	6.20%	[1]	0.80%	[1]
Effective Income Tax Rate Reconciliation, Other Regulatory Items, Percent		(1.70%)	(1.70%)	[1]	(1.00%)	[1]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent		0.50%	0.40%	[1]	(0.60%)	[1]
Tax reform, Percent		0.00%	0.00%	[1]	1.40%	[1]
Effective Income Tax Rate Reconciliation Change In Unrecognized Tax Benefits		(1.00%)	(0.70%)	[1]	(1.30%)	[1]
Effective Income Tax Rate Reconciliation, Percent		8.50%	12.60%	[1]	32.10%	[1]

[1] Prior periods have been reclassified to conform to current year presentation.

[2] Regulatory differences for income tax primarily relate to the credit of excess deferred taxes to customers through the average rate assumption method. Income tax benefits associated with the credit of excess deferred credits are offset by corresponding revenue reductions and additional prepaid pension asset amortization.