Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2019	Dec. 31, 2018
Supplemental cash flow information - Noncash activities
Trading debt securities retained from securitization of MLHFS	$ 31,517	$ 28,761
Transfers from loans to MLHFS	5,409	4,456
Transfers from loans to LHFS	117	2,542
Transfers from available-for-sale debt securities to held-to-maturity debt securities	13,833	13,372
Operating lease ROU assets acquired with operating lease liabilities	5,644	$ 0
Summary Of Significant Accounting Policies Textuals [Abstract]
Operating lease ROU assets (lessee)	4,856			$ 0
Lease liabilities	$ 5,383
Direct financing lease, risk strategy, residual asset	Our allowance for loan losses for financing leases considers both the collectability of the lease payments receivable as well as the estimated residual value of the leased asset. We typically purchase residual value insurance on our financing leases so that our risk of loss at lease termination will be less than 10% of the initial value of the lease. Our risk to declines in residual values is further mitigated by the diversity of leased assets in our lease portfolio. In addition, we have several channels for re-leasing or marketing those assets.
Operating lease ROU assets acquired with operating lease liabilities	$ 744
Private Forward Repurchase Transaction [Member]
Summary Of Significant Accounting Policies Textuals [Abstract]
Private forward repurchase contract	$ 0
Accounting Standards Update 2017-08 [Member]
Summary Of Significant Accounting Policies Textuals [Abstract]
Cumulative effect of new accounting principle			$ (111)
Accounting Standards Update 2017-08 [Member] | Cumulative other comprehensive income [Member]
Summary Of Significant Accounting Policies Textuals [Abstract]
Cumulative effect of new accounting principle				$ 481
Accounting Standards Update 2017-08 [Member] | Retained earnings [Member]
Summary Of Significant Accounting Policies Textuals [Abstract]
Cumulative effect of new accounting principle			(592)
Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies Textuals [Abstract]
Operating lease ROU assets (lessee)			4,900
Lease liabilities			5,600
Accounting Standards Update 2016-02 [Member] | Retained earnings [Member]
Summary Of Significant Accounting Policies Textuals [Abstract]
Cumulative effect of new accounting principle			$ 100