Adoption of New Leasing Standard - Lessee - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Leases [Abstract]
Remaining terms of leases	10 years	10 years
Operating lease, not yet commenced, obligation	$ 23	$ 23
Operating lease, not yet commenced, ROU asset	$ 23	$ 23
Operating lease, not yet commenced, term of lease	10 years	10 years
Remaining lease obligation	$ 7	$ 7
Finance leases, discount rate	4.07%	4.07%
Finance lease, right-of-use asset	$ 7	$ 7
Operating lease expense	$ 31	$ 97	$ 147
Net of sublease income				$ 161