Transfers of Receivables - Financial Performance Related to Securitizations (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Securitization Transactions [Line Items]
Interest expense	$ 867	$ 810	$ 769	$ 729	$ 725	$ 697	$ 687	$ 646	$ 3,175	$ 2,755	$ 2,416
Securitization Transactions [Member]
Securitization Transactions [Line Items]
Interest expense									955	773	630
Variable Interest Entity, Primary Beneficiary [Member] | Securitization Transactions [Member]
Securitization Transactions [Line Items]
Interest expense									827	671	541
Consolidated Entity Excluding Variable Interest Entities (VIE) [Member] | Securitization Transactions [Member]
Securitization Transactions [Line Items]
Interest expense									$ 128	$ 102	$ 89