Leases - Lease Cost and Additional Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Operating lease cost	$ 82
Short-term lease cost	1
Variable lease cost	37
Sublease income	(4)
Net lease cost	116
Operating cash outflows from operating leases	87
Right of use assets obtained in exchange for new operating lease liabilities	$ 0
Real Estate Leases
Lessee, Lease, Description [Line Items]
Weighted-average remaining lease term	7 years 8 months 12 days
Weighted-average discount rate	3.12%