Leases Leases - (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Lessee, Lease, Description [Line Items]
Operating Leases, Rent Expense, Net			$ 342
Sublease income	$ 99	$ 89	$ 72
Minimum
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Term of Contract	1 year
Maximum
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Term of Contract	14 years
Sublease Income | Minimum
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Term of Contract	1 year
Sublease Income | Maximum
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Term of Contract	9 years