Description of Business, Basis of Presentation, and Changes in Significant Accounting Policies (Details) - USD ($) $ in Millions	Sep. 30, 2018	Jan. 01, 2018
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of changes in accounting principles, net of tax	$ 126	$ 126
Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of changes in accounting principles, net of tax	20	(7)
Accounting Standards Update 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of changes in accounting principles, net of tax	$ (42)	$ 0