Variable Interest Entities - Multi-seller conduits (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Variable Interest Entity [Line Items]
Unfunded lending-related commitments	$ 1,048,674	$ 991,482	$ 999,817
Multi-seller conduits
Variable Interest Entity [Line Items]
Commercial paper eliminated in consolidation	18,700	20,400
Multi-seller conduits | Commercial and other
Variable Interest Entity [Line Items]
Unfunded lending-related commitments	$ 9,200	$ 8,800