Summary of Significant Accounting Policies - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) payment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Summary of Significant Accounting Policies [Line Items]
Probable loss experience period	10 years
Number of delinquent payments that can be moved | payment	2
Net cash used in operating activities	$ (6,752)	$ (4,881)	$ (2,168)
Net cash used in investing activities	21,856	24,774	22,033
Beginning of period cash and cash equivalents	$ 6,567	$ 5,302	$ 5,002	$ 5,045
Minimum [Member]
Summary of Significant Accounting Policies [Line Items]
Credit facility expiration period	12 months
Estimated useful life (in years)	1 year
Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Credit facility expiration period	18 months
Estimated useful life (in years)	30 years
Retail Finance Receivables [Member]
Summary of Significant Accounting Policies [Line Items]
Contractually delinquent period	120 days
Commercial Finance Receivables [Member]
Summary of Significant Accounting Policies [Line Items]
Typical repayment period	90 days
Contractually delinquent period	360 days
Commercial Finance Receivables [Member] | Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Payment period after loan is called	60 days