Leases - Components of lease expense (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 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	$ (17)	$ (50)
Variable lease expense	3	(7)
Sublease income	1	3
Net lease expense	$ (13)	$ (54)