Leases - Remaining Lease Term And Lease Expense (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Jan. 01, 2019
Leases
Lease, Practical Expedients, Package [true false]	true
Lease, Practical Expedient, Use of Hindsight [true false]	false
Operating lease ROU assets (1)	$ 343	$ 287
Short term operating lease liabilities	52	51
Long term operating lease liabilities	$ 317	259
Prepaid rent		$ 23
Operating lease, existence of option to extend	true
Finance lease, existence of option to extend	true
Operating lease, existence of option to terminate	true
Finance lease, existence of option to terminate	true
Components of lease expense:
Operating Lease Cost	$ 17
Finance lease cost
Depreciation of leased assets	5
Interest on lease liabilities	2
Total finance lease cost	$ 7
Minimum
Leases
Operating lease, remaining lease term	1 year
Finance lease, remaining lease term	1 year
Maximum
Leases
Operating lease, remaining lease term	15 years
Finance lease, remaining lease term	15 years
Operating lease, option to extend, period	14 years
Finance lease, option to extend, period	14 years
Operating lease, terminate term	1 year
Finance lease, terminate term	1 year