Leases - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Leases [Abstract]
Operating lease expense	$ 37	$ 38
Right-of-use assets obtained in exchange for operating lease obligations	$ 6
Weighted-average remaining lease term (years)	6 years
Weighted-average discount rate	6.20%