Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	$ 7,592
Accumulated other comprehensive loss at end of period	7,945
Income Tax Expense (Benefit)	11	$ 17
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(23)	(24)
Net current period other comprehensive loss	1	0
Accumulated other comprehensive loss at end of period	(22)	(24)
Gains and Losses on Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Income Tax Expense (Benefit)	0	0
Gains and Losses on Cash Flow Hedges | Interest Rate Swap
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Losses reclassified from net accumulated other comprehensive loss	(1)	0
Defined Benefit and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(1)	(3)
Net current period other comprehensive loss	0	0
Accumulated other comprehensive loss at end of period	(1)	(3)
Defined Benefit and Postretirement Items | Interest Rate Swap
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Losses reclassified from net accumulated other comprehensive loss	0	0
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(24)	(27)
Net current period other comprehensive loss	1	0
Accumulated other comprehensive loss at end of period	(23)	(27)
Total | Interest Rate Swap
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Losses reclassified from net accumulated other comprehensive loss	$ (1)	$ 0