Summary of Significant Accounting Policies - Operating Lease Details (Details) - USD ($) $ in Millions	Jun. 30, 2019	Jan. 01, 2019
Accounting Policies [Abstract]
Right-of-use lease assets	$ 301	$ 308
Current operating lease liability	54	52
Non-current operating lease liability	298	305
Total operating lease liability	$ 352	$ 357