Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Effective tax rate on income from continuing operations (as a percent)	15.70%	23.10%	15.20%	19.30%
Valuation allowance	$ 17		$ 17		$ 17
Gross unrecognized tax benefits	85		85		76
Unrecognized tax benefits net of federal tax benefits that would affect the effective tax rate	65		65		58
Increase (decrease) in interest and penalties	0	$ 0	1	$ 1
Payable related to accrued interest and penalties	9		9		$ 8
Minimum [Member]
Operating Loss Carryforwards [Line Items]
Decrease in gross unrecognized tax benefits due to resolution of audits and statute expirations	40		40
Maximum [Member]
Operating Loss Carryforwards [Line Items]
Decrease in gross unrecognized tax benefits due to resolution of audits and statute expirations	50		50
State and Local [Member]
Operating Loss Carryforwards [Line Items]
State net operating losses	$ 18		$ 18
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