Leases - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Leases [Abstract]
Rental income	$ 51	$ 53	$ 101	$ 96
Percent of operating revenue	2.50%	2.60%	2.50%	2.30%