Summary of Significant Accounting Policies - Maturities of Operating Lease Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2019	Jan. 01, 2019
Accounting Policies [Abstract]
Remainder of 2019	$ 33
2020	64
2021	63
2022	60
2023	44
Thereafter	136
Lease liability amounts repayable	400
Interest costs	48
Total operating lease liability	$ 352	$ 357