Basis of Presentation and Significant Accounting Policies - Adoption of New Accounting Pronouncements and Accounting Pronouncements Issued but Not Yet Effective (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Jan. 01, 2019
Accounting Policies [Abstract]
Tax cuts and jobs act of 2017, provisional amount income tax benefit	$ 173	$ 173
Accounting Standards Update 2016-02 | Scenario, Forecast
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Right-of-use asset			$ 2,000
Operating lease liability			$ 2,000