Loans, Lending Commitments and Allowance for Credit Losses - Loans Held for Investment before Allowance by Credit Quality (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	$ 105,410	$ 100,053
Corporate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	40,283	36,909
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	29,196	27,868
Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	28,595	27,466
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	7,336	7,810
Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	104,388	98,845
Pass | Corporate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	39,775	36,217
Pass | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	29,191	27,863
Pass | Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	28,490	27,387
Pass | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	6,932	7,378
Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	388	809
Special mention | Corporate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	94	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	10	0
Special mention | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	284	312
Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	634	399
Substandard | Corporate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	414	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	95	79
Substandard | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, gross	120	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