Leases - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Leases [Abstract]
Operating lease expense	$ 36	$ 35	$ 110	$ 109
Right-of-use assets obtained in exchange for operating lease obligations	$ 7		$ 25
Weighted-average remaining lease term	6 years		6 years
Weighted-average discount rate	6.10%		6.10%