Income Taxes - Effective Income Tax Reconciliation (Parenthetical) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Income Tax Examination [Line Items]
Total windfall benefits on stock-based compensation	$ (22)
U.S Federal [Member]
Income Tax Examination [Line Items]
Total windfall benefits on stock-based compensation	(20)
State [Member]
Income Tax Examination [Line Items]
Total windfall benefits on stock-based compensation	$ (2)