Transfers of Receivables - Financial Performance Related to Securitizations (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Securitization Transactions [Line Items]
Interest expense	$ 1,032	$ 989	$ 997	$ 912	$ 867	$ 810	$ 769	$ 729	$ 3,930	$ 3,175	$ 2,755
Securitization Transactions [Member]
Securitization Transactions [Line Items]
Interest expense									1,397	955	773
Variable Interest Entity, Primary Beneficiary [Member] | Securitization Transactions [Member]
Securitization Transactions [Line Items]
Interest expense									1,220	827	671
Consolidated Entity Excluding Variable Interest Entities (VIE) [Member] | Securitization Transactions [Member]
Securitization Transactions [Line Items]
Interest expense									$ 177	$ 128	$ 102