Summary of Significant Accounting Policies - Recent Accounting Developments (Details) - USD ($) $ in Millions	Jun. 30, 2019	Jan. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease liabilities	$ 587
Operating lease right-of-use assets	$ 499
ASU 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease liabilities		$ 404
Operating lease right-of-use assets		$ 321