Leases - Lease Costs (Details) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)
Leases [Abstract]
Operating lease expense	$ 30	$ 64
Short-term lease expense	2	4
Variable lease expense	12	20
Total lease expense	44	88
Operating cash flows from operating leases	$ 41	$ 82
Weighted average remaining lease term, operating lease	4 years 11 months 15 days	4 years 11 months 15 days
Weighted average remaining lease term, finance lease	3 years 18 days	3 years 18 days
Weighted average discount rate, operating lease	4.26%	4.26%
Weighted average discount rate, finance lease	5.08%	5.08%