Basis Of Presentation Financial Instruments - Fair Value Measurements (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Liabilities Rollforward [Roll Forward]
Fair value of contingent consideration at December 31, 2019	$ 4
Additions: contingent consideration related to acquisitions completed during the period	1
Reductions: payments of contingent consideration	0
Changes in fair value (reflected in general and administrative expenses)	0
Fair value of contingent consideration at March 31, 2020	5
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 current and non-current liabilities)	96	47
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 current and 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 current and non-current liabilities)	96	47
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 current and 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)	5	4
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)	$ 5	$ 4