Leases - Lease Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Leases [Abstract]
Operating lease expense	$ 122	$ 131
Amortization of right-of-use assets	2	1
Short-term lease expense	6	13
Variable lease expense	24	26
Sublease income	(1)	(1)
Total lease expense	$ 153	$ 170
Rent expense			$ 111
Weighted average remaining lease term, operating leases	5 years 4 months 17 days	4 years 9 months 25 days
Weighted average remaining lease term, finance leases	3 years 11 months 19 days	3 years 2 months 4 days
Weighted average discount rate, operating leases	3.63%	4.24%
Weighted average discount rate, finance leases	3.07%	4.02%