Summary of Significant Accounting Policies - Recent Accounting Developments (Details) - USD ($) $ in Millions |
Mar. 31, 2019 |
Jan. 01, 2019 |
Dec. 31, 2018 |
---|---|---|---|
New Accounting Pronouncements or Change in Accounting Principle [Line Items] | |||
Operating lease liabilities | $ 603 | ||
Operating lease right-of-use assets | $ 517 | $ 0 | |
ASU 2016-02 | |||
New Accounting Pronouncements or Change in Accounting Principle [Line Items] | |||
Operating lease liabilities | $ 404 | ||
Operating lease right-of-use assets | $ 321 |