Basis Of Presentation Financial Instruments - Fair Value Measurements (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value of contingent consideration at December 31, 2016	$ 50
Additions: contingent consideration related to acquisitions completed during the period	3
Reductions: payments of contingent consideration (reflected in the financing section of the Consolidated Statement of Cash Flows)	(18)
Changes in fair value (reflected in the Consolidated Statement of Operations)	(2)
Fair value of contingent consideration at September 30, 2017	33
Deferred Compensation Plan Assets | Fair Value, Measurements, Recurring
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 | Fair Value, Measurements, Recurring | 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 | Fair Value, Measurements, Recurring | 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 | Fair Value, Measurements, Recurring | 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
Contingent Consideration for Acquisitions | Fair Value, Measurements, Recurring
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 non-current liabilities)	33	50
Contingent Consideration for Acquisitions | Fair Value, Measurements, Recurring | 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 non-current liabilities)	0	0
Contingent Consideration for Acquisitions | Fair Value, Measurements, Recurring | 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 non-current liabilities)	0	0
Contingent Consideration for Acquisitions | Fair Value, Measurements, Recurring | 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 non-current liabilities)	33	50
Interest Rate Swap | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps (included in other current and non-current liabilities)	24	33
Interest Rate Swap | Fair Value, Measurements, Recurring | Level I
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps (included in other current and non-current liabilities)	0	0
Interest Rate Swap | Fair Value, Measurements, Recurring | Level II
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps (included in other current and non-current liabilities)	24	33
Interest Rate Swap | Fair Value, Measurements, Recurring | Level III
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps (included in other current and non-current liabilities)	$ 0	$ 0