Financial Instruments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Liabilities Rollforward [Roll Forward]
Fair value of contingent consideration at December 31, 2017	$ 34
Additions: contingent consideration related to acquisitions completed during the period	1
Reductions: payments of contingent consideration	(23)
Changes in fair value (reflected in the Consolidated Statement of Operations)	(2)
Fair value of contingent consideration at December 31, 2018	10
Deferred compensation plan assets (included in other non-current assets) | Fair Value Measurements Recurring Member
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation plan assets (included in other non-current assets)	2	$ 3
Deferred compensation plan assets (included in other non-current assets) | Fair Value Measurements Recurring Member | 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	3
Deferred compensation plan assets (included in other non-current assets) | Fair Value Measurements Recurring Member | 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 (included in other non-current assets) | Fair Value Measurements Recurring Member | 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 contracts | Fair Value Measurements Recurring Member
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)	16	13
Interest rate swap contracts | Fair Value Measurements Recurring Member | 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
Interest rate swap contracts | Fair Value Measurements Recurring Member | 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)	16	13
Interest rate swap contracts | Fair Value Measurements Recurring Member | 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
Contingent consideration for acquisitions (included in accrued expenses and other current liabilities and other non-current liabilities) | Fair Value Measurements Recurring Member
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)	10	34
Contingent consideration for acquisitions (included in accrued expenses and other current liabilities and other non-current liabilities) | Fair Value Measurements Recurring Member | 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 (included in accrued expenses and other current liabilities and other non-current liabilities) | Fair Value Measurements Recurring Member | 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 (included in accrued expenses and other current liabilities and other non-current liabilities) | Fair Value Measurements Recurring Member | 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)	$ 10	$ 34