Leases - Lease Cost and Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Lessee, Lease, Description [Line Items]
Operating lease cost	$ 99	$ 94	$ 288	$ 276
Short-term lease cost	1	4	3	7
Variable lease cost	28	37	92	113
Sublease income	(5)	(4)	(15)	(12)
Net lease cost	$ 123	$ 131	368	384
Operating cash outflows from operating leases			306	288
Right of use assets obtained in exchange for new operating lease liabilities			$ 162	$ 112
Real Estate Lease
Lessee, Lease, Description [Line Items]
Weighted-average remaining lease term	8 years 5 months 8 days	8 years 10 months 9 days	8 years 5 months 8 days	8 years 10 months 9 days
Weighted-average discount rate	3.02%	3.09%	3.02%	3.09%