Summary of Significant Accounting Policies - Maturities of Operating Lease Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2019	Jan. 01, 2019
Accounting Policies [Abstract]
Remainder of 2019	$ 49
2020	63
2021	61
2022	59
2023	43
Thereafter	134
Lease liability amounts repayable	409
Interest costs	50
Total operating lease liability	$ 359	$ 357