Leases - Components of Operating Lease Costs (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Leases [Abstract]
Operating lease cost	$ 29	$ 95
Variable lease cost	15	46
Short-term lease cost	5	11
Sublease income	(16)	(44)
Total operating lease costs, net	33	108
Cash paid for amounts included in the measurement of operating lease liabilities	30	92
ROU assets recognized for new operating lease liabilities	$ 72	$ 82