Leases - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2019
Lessee, Lease, Description [Line Items]
Operating lease, option to extend term	5 years
Finance lease, option to extend term	5 years
Operating lease, termination period	1 year
Finance lease, termination period	1 year
Assets under finance leases, gross	$ 82	$ 82
Accumulated depreciation under finance leases	12	9
Liabilities arising from capital lease transactions	24	25		$ 70
Rent expense	9	12
Rent expense			$ 57
Right-of-use assets	$ 223	$ 226
Minimum
Lessee, Lease, Description [Line Items]
Operating Lease, Remaining Lease Term	1 year
Finance Lease, Remaining Lease Term	1 year
Maximum
Lessee, Lease, Description [Line Items]
Operating Lease, Remaining Lease Term	30 years
Finance Lease, Remaining Lease Term	30 years
Land | Minimum
Lessee, Lease, Description [Line Items]
Operating Lease, Remaining Lease Term	48 years
Land | Maximum
Lessee, Lease, Description [Line Items]
Operating Lease, Remaining Lease Term	90 years