Significant Accounting Policies - Factored Receivables (Details) - USD ($) $ in Billions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Significant Accounting Policies
Gross proceeds from factored receivables	$ 2.2	$ 1.9
Accounts receivable sold and derecognized that remain uncollected from customers	0.9	0.7
Net gains and losses associated with the transfer of receivables

Reflects the net positive or negative amount derived from subtracting from net proceeds of sale, the carrying amounts, net of allocated reserves, of accounts receivable transferred to a third party in a transaction that qualifies for sales treatment.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=109247956&loc=d3e107425-111719

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration