Summary of Significant Accounting Policies - Maturities of Operating Lease Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2019	Jan. 01, 2019
Accounting Policies [Abstract]
Remainder of 2019	$ 14
2020	64
2021	63
2022	61
2023	44
Thereafter	137
Lease liability amounts repayable	383
Interest costs	44
Total operating lease liability	$ 339	$ 357