Other Assets - Affordable Housing and Other Tax Credit Investment Partnerships (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025	Dec. 31, 2025
Affordable housing and other tax credit investment partnership interests
Investment partnership interest	$ 297.8		$ 297.8		$ 302.3
Investment, Proportional Amortization Method, Elected, Statement of Financial Position [Extensible Enumeration]	Other Assets		Other Assets		Other Assets
Amortization	$ 8.9	$ 7.6	$ 17.9	$ 15.2
Investment Program, Proportional Amortization Method, Applied, Amortization Expense, Statement of Income or Comprehensive Income [Extensible Enumeration]	Income Tax Expense (Benefit)	Income Tax Expense (Benefit)	Income Tax Expense (Benefit)	Income Tax Expense (Benefit)
Affordable housing tax credits and other benefits recognized	$ 12.0	$ 9.6	$ 23.9	$ 19.3
Investment Program, Proportional Amortization Method, Elected, Income Tax Credit and Other Income Tax Benefit, before Amortization, Statement of Income or Comprehensive Income [Extensible Enumeration]	Income Tax Expense (Benefit)	Income Tax Expense (Benefit)	Income Tax Expense (Benefit)	Income Tax Expense (Benefit)
Investment Program, Proportional Amortization Method, Elected, Income Tax Credit and Other Income Tax Benefit, before Amortization, Statement of Cash Flows [Extensible Enumeration]	Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	Net Income (Loss), Including Portion Attributable to Noncontrolling Interest
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Affordable housing and other tax credit investment partnership interests
Unfunded commitments to fund low-income housing partnerships					$ 153.3
Location, Statement of Financial Position, Balance [Axis]: fhb:FsTypedDimensionConstraint3Member \| Variable Interest Entity, Not Primary Beneficiary
Affordable housing and other tax credit investment partnership interests
Unfunded commitments to fund low-income housing partnerships	$ 145.2		$ 145.2

Amount, before amortization expense, of income tax credit and other tax benefit generated by investment under tax credit program elected to be accounted for using proportional amortization method. Includes, but is not limited to, investment under tax credit program not meeting condition for accounting under proportional amortization method.

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