Other Comprehensive Income (Reclassifications from AOCI) (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017		Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	$ (319)	$ (329)		$ (670)	$ (686)
Income Tax Expense (Benefit)	54	102		114	219
Total amounts reclassified, net of tax	2	2		3	3
Gains and Losses on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amounts reclassified, net of tax	1	1		1	1
Gains and Losses on Cash Flow Hedges | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	1	2		1	2
Income Tax Expense (Benefit)	0	(1)		0	(1)
Total, net of tax	1	1		1	1
Gains and Losses on Cash Flow Hedges | Interest rate derivatives | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	1	2	[1]	1	2	[1]
Amortization of net loss | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	2	2		3	3
Defined Benefit Pension and Postretirement Items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amounts reclassified, net of tax	1	1		2	2
Defined Benefit Pension and Postretirement Items | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	2	2		3	3
Tax benefit	(1)	(1)		(1)	(1)
Total amounts reclassified, net of tax	$ 1	$ 1		$ 2	$ 2

[1] Included in interest charges