Leases - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Apr. 30, 2018
Lessee, Lease, Description [Line Items]
Variable lease cost	$ 380	$ 327
Lease liability, future minimum commitment	$ 1,618
Operating leases, rent expense, net			$ 301
Office Building In New York City [Member]
Lessee, Lease, Description [Line Items]
Operating lease term				20 years
Lease liability, future minimum commitment				$ 1,600
Minimum [Member]
Lessee, Lease, Description [Line Items]
Operating lease term	1 year
Operating lease term, option to extend	5 years
Maximum [Member]
Lessee, Lease, Description [Line Items]
Operating lease term	30 years
Operating lease term, option to extend	10 years