Postretirement Benefit Plans (Components of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure
Net loss	$ 78	$ 83	$ 37
Total recognized in comprehensive income	(2)	0	0
Other Benefits
Defined Benefit Plan Disclosure
Net loss	1	(2)	1
Defined Benefit Plan, Prior Service Credit	(5)	0	(3)
Amortization of prior service credit	3	4	4
Amortization of net actuarial loss	(2)	(2)	(2)
Change recognized in other comprehensive loss	$ (3)	$ 0	$ 0