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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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Percentage of plan assets to benefit obligation of defined benefit plan.

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Number of shares of employer stock in which defined benefit plan asset is invested. Includes, but is not limited to, stock issued or managed by related party of employer.

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Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

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Amount of cost for defined contribution plan.

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Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

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Percentage employer matches of the employee's percentage contribution matched.

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Percentage of employer's matching contributions to a defined contribution plan that vests in a given year.

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Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

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Amount of paid and unpaid common stock dividends declared with the form of settlement in cash, stock and payment-in-kind (PIK).

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Employee Benefit Plans - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2025	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Defined Benefit Plan Disclosure [Line Items]
Unrecognized actuarial gains (losses), before tax		$ 10,725,000,000
Amount of United common stock held in the pension plan		$ 197,381,000	$ 183,035,000	$ 173,840,000
Percentage of projected benefit obligation		10.00%
Minimum number of days required to be eligible for the participation in the plan		90 days
Percentage of salary deferred under condition one of contribution by company	100.00%
First percentage of salary deferred under condition one of contribution by company	5.00%
Vesting percentage of employee deferrals	100.00%
Cost related to savings and stock investment plan		$ 7,829,000	$ 7,332,000	7,590,000
Employer contribution in shares of common stock		1,879,671	1,810,400
Dividends paid on United common stock		$ 2,755,000	$ 2,727,000	$ 2,468,000
Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Maximum percentage allowed as of contribution to respective accounts by participants	1.00%
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Maximum percentage allowed as of contribution to respective accounts by participants	100.00%
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Amount of United common stock held in the pension plan		$ 4,064,000	3,974,000
Maximum percentage limit of common stock to invest in portfolio for any industry		47.00%
Approximate fair value of plan assets		$ 72,179,000	$ 67,981,000
Equity Securities [Member] \| Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Maximum percentage limit of common stock to invest in portfolio for any industry		20.00%
Equity Securities [Member] \| Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Maximum percentage limit of common stock to invest in portfolio for any industry		70.00%
Individual Company [Member] \| Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Maximum percentage limit of common stock to invest in portfolio for any industry		10.00%
Industry [Member] \| Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Maximum percentage limit of common stock to invest in portfolio for any industry		15.00%
Securities of U.S. Government or Agencies [Member] \| Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Maximum percentage limit of common stock to invest in portfolio for any industry		15.00%