Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Effective tax rate on income from continuing operations (as a percent)	14.70%	15.20%
Valuation allowance	$ 16		$ 17
Gross unrecognized tax benefits	83		76
Unrecognized tax benefits net of federal tax benefits that would affect the effective tax rate	59		58
Increase (decrease) in interest and penalties	1
Payable related to accrued interest and penalties	9		$ 8
Minimum [Member]
Operating Loss Carryforwards [Line Items]
Decrease in gross unrecognized tax benefits due to resolution of audits and statute expirations	40
Maximum [Member]
Operating Loss Carryforwards [Line Items]
Decrease in gross unrecognized tax benefits due to resolution of audits and statute expirations	50
State and Local [Member]
Operating Loss Carryforwards [Line Items]
State net operating losses	$ 17
Income Tax Provision [Member]
Operating Loss Carryforwards [Line Items]
Benefit related to an out-of-period correction for a reversal of a tax reserve		$ 20