Pension - Components of Net Pension Cost (Details) - Pension Plan - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost			$ 2	$ 2	$ 3
Interest cost			168	194	169
Expected long-term return on plan assets			(260)	(245)	(257)
Amortization of:
Prior actuarial losses, net			78	59	70
Prior service cost			0	0	0
Litigation settlement - plan amendment	$ 142	$ 32	0	142	32
Settlement loss			0	10	0
Net (benefit) cost			$ (12)	$ 162	$ 17