Leases - Components of lease expense (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Leases
Lessee, Operating Lease, Existence of Option to Extend [true false]		true
Lessee, Operating Lease, Existence of Option to Terminate [true false]		true
Components of lease expense
Operating lease expense	$ (16)	$ (33)
Variable lease expense	(8)	(10)
Sublease income	1	2
Net lease expense	$ (23)	$ (41)