Basis Of Presentation Financial Instruments - Fair Value Measurements (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Deferred Compensation Plan Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation plan assets (included in other non-current assets)	$ 3	$ 3
Deferred Compensation Plan Assets \| Level I
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation plan assets (included in other non-current assets)	3	3
Deferred Compensation Plan Assets \| Level II
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation plan assets (included in other non-current assets)	0	0
Deferred Compensation Plan Assets \| Level III
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation plan assets (included in other non-current assets)	0	0
Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps (included in other non-current assets)	9
Interest rate swaps (included in other current and non-current liabilities)	6	13
Interest Rate Swap \| Level I
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps (included in other non-current assets)	0
Interest rate swaps (included in other current and non-current liabilities)	0	0
Interest Rate Swap \| Level II
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps (included in other non-current assets)	9
Interest rate swaps (included in other current and non-current liabilities)	6	13
Interest Rate Swap \| Level III
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps (included in other non-current assets)	0
Interest rate swaps (included in other current and non-current liabilities)	0	0
Contingent Consideration for Acquisitions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration for acquisitions (included in accrued expenses and other current liabilities and other non-current liabilities)	34	34
Contingent Consideration for Acquisitions \| Level I
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration for acquisitions (included in accrued expenses and other current liabilities and other non-current liabilities)	0	0
Contingent Consideration for Acquisitions \| Level II
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration for acquisitions (included in accrued expenses and other current liabilities and other non-current liabilities)	0	0
Contingent Consideration for Acquisitions \| Level III
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration for acquisitions (included in accrued expenses and other current liabilities and other non-current liabilities)	$ 34	$ 34