Receivables (Details)		6 Months Ended		12 Months Ended
	Mar. 30, 2012 USD ($)	Jun. 30, 2018 USD ($) agreement	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule Of Financial Receivables [Line Items]
Number of arrangement with different financial institution to sell customer receivables | agreement		3
Funding of customer receivables sold		$ 0
Sale of customer receivables	$ 304,000,000
Cash received for receivables	50,000,000	420,000,000	$ 285,000,000
Deferred purchase program receivable	$ 254,000,000	313,000,000		$ 187,000,000
Net cash funding received during the period		300,000,000
Amount of cash draws under arrangement during the period		2,658,000,000
Amount of cash repayments under arrangement during the period		(2,358,000,000)
Amount of cash draws under arrangement since inception		(300,000,000)
Amount of cash repayments under arrangement since inception		300,000,000
Cash outflow from deferred purchase receivable		38,547,000,000
Cash collections of other receivables		(37,885,000,000)
Minimum
Schedule Of Financial Receivables [Line Items]
Funding of customer receivables sold		200,000,000
Maximum
Schedule Of Financial Receivables [Line Items]
Funding of customer receivables sold		$ 400,000,000