Accumulated Other Comprehensive Income - Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 6,975	$ 6,814
Amounts reclassified from AOCI	(23)	(6)
Other comprehensive loss	(23)	(6)
Income taxes (benefits) on other comprehensive income (loss)	(5)	(1)
Other comprehensive loss, net of tax	(18)	(5)
Ending balance	6,814	6,932
Gains & Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(9)	(11)
Amounts reclassified from AOCI	0	1
Other comprehensive loss	0	1
Income taxes (benefits) on other comprehensive income (loss)	0	0
Other comprehensive loss, net of tax	0	1
Ending balance	(9)	(10)
Defined Benefit Pension & OPEB Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	29	52
Amounts reclassified from AOCI	(23)	(7)
Other comprehensive loss	(23)	(7)
Income taxes (benefits) on other comprehensive income (loss)	(5)	(1)
Other comprehensive loss, net of tax	(18)	(6)
Ending balance	11	46
AOCI
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	20	41
Other comprehensive loss, net of tax	(18)	(5)
Ending balance	$ 2	$ 36