Receivables (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 28, 2019
Receivables [Abstract]
Client cash collections	$ 5,700	$ 5,790
Accounts Receivable, after Allowance for Credit Loss, Noncurrent	$ 17	$ 47
Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Description	In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which provides a new impairment model for certain financial assets that is based on expected losses rather than incurred losses. Such guidance impacts how we determine our allowance for estimated uncollectible client receivables. The standard requires use of the modified retrospective (cumulative effect) transition approach as of the beginning of the first reporting period in which the guidance was effective, which for the Company was the first quarter of 2020. Under this transition method, the cumulative effect from prior periods upon applying this new guidance was recognized in our consolidated balance sheets as of December 29, 2019. We did not recast comparative periods.