Summary of Significant Accounting Policies - Operating Lease Details (Details) - USD ($) $ in Millions	Sep. 30, 2019	Jan. 01, 2019
Accounting Policies [Abstract]
Right-of-use lease assets	$ 292	$ 308
Current operating lease liability	52	52
Non-current operating lease liability	287	305
Total operating lease liability	$ 339	$ 357