Loans, Lending Commitments and Allowance for Credit Losses - Loans Held for Investment before Allowance by Credit Quality (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	$ 111,112	$ 100,053
Corporate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	45,117	36,909
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	30,010	27,868
Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	29,184	27,466
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	6,801	7,810
Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	109,826	98,845
Pass | Corporate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	44,152	36,217
Pass | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	30,005	27,863
Pass | Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	29,051	27,387
Pass | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	6,618	7,378
Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	480	809
Special mention | Corporate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	373	492
Special mention | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	0	5
Special mention | Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	27	0
Special mention | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	80	312
Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	806	399
Substandard | Corporate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	592	200
Substandard | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	5	0
Substandard | Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	106	79
Substandard | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	103	120
Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	0	0
Doubtful | Corporate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	0	0
Doubtful | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	0	0
Doubtful | Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	0	0
Doubtful | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	0	0
Loss
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	0	0
Loss | Corporate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	0	0
Loss | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	0	0
Loss | Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	0	0
Loss | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	$ 0	$ 0