Significant Accounting Policies - Factored Receivables (Details) - USD ($) $ in Billions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Significant Accounting Policies
Gross proceeds from factored receivables	$ 2.1	$ 2.2
Accounts receivable sold and derecognized that remain uncollected from customers	0.5	0.9
Net gains and losses associated with the transfer of receivables

Amount of gain (loss) on sale of accounts receivable.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118948506&loc=d3e107425-111719

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