Retirement and Benefit Plans - Summary of plans (Details) - USD ($) $ in Millions	Mar. 31, 2026	Feb. 01, 2026
Postemployment Retirement Benefits \| Acquired Comerica Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 42
Accumulated benefit obligation	13
Funded status	29
Qualified Plan \| Pension Plan \| Acquired Comerica Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,970
Projected benefit obligation	1,615
Accumulated benefit obligation	1,595
Funded status	1,355
Nonqualified Plan \| Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 0
Nonqualified Plan \| Pension Plan \| Acquired Comerica Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0
Projected benefit obligation	158
Accumulated benefit obligation	154
Funded status	$ (158)

Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level.

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Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

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Amount of funded (unfunded) status of defined benefit plan, measured as difference between fair value of plan assets and benefit obligation. Includes, but is not limited to, overfunded (underfunded) status.

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