Leases - Components of lease expense (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
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)
Variable lease expense	(2)
Sublease income	1
Net lease expense	$ (18)