Basis Of Presentation Financial Instruments - Fair Value Measurements (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Fair Value, Liabilities Rollforward [Roll Forward]
Fair value of contingent consideration at December 31, 2018	$ 10
Additions: contingent consideration related to acquisitions completed during the period	0
Reductions: payments of contingent consideration	(4)
Changes in fair value (reflected in general and administrative expenses)	(2)
Fair value of contingent consideration at September 30, 2019	4
Fair Value, Measurements, Recurring \| 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)	2	$ 2
Fair Value, Measurements, Recurring \| 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)	2	2
Fair Value, Measurements, Recurring \| 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
Fair Value, Measurements, Recurring \| 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
Fair Value, Measurements, Recurring \| 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)		6
Interest rate swaps (included in other non-current liabilities)	60	16
Fair Value, Measurements, Recurring \| 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 non-current liabilities)	0	0
Fair Value, Measurements, Recurring \| 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)		6
Interest rate swaps (included in other non-current liabilities)	60	16
Fair Value, Measurements, Recurring \| 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 non-current liabilities)	0	0
Fair Value, Measurements, Recurring \| 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)	4	10
Fair Value, Measurements, Recurring \| 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
Fair Value, Measurements, Recurring \| 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
Fair Value, Measurements, Recurring \| 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)	$ 4	$ 10

Fair Value, Liabilities Rollforward [Roll Forward]

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of gain (loss) recognized in income from liability measured at fair value on recurring basis using unobservable input (level 3).

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- Definition

Amount of issuances of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of settlements of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Fair value portion of other assets.

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Fair value portion of other liabilities.

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