Employee Benefit Plans - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Net periodic pension cost decline in expense		$ 1,000,000
Unrecognized actuarial gains (losses), before tax		56,222,000
Unrecognized actuarial gains (losses), net of tax		35,420,000
Amortization expected to be recognized		4,653,000
Amortization expected to be recognized, net of tax		2,932,000
Amount of United common stock held in the pension plan		$ 142,395,000	$ 119,711,000	$ 113,748,000
Percentage of equity securities include united common stock		100.00%	100.00%
Percentage of projected benefit obligation		10.00%
Employer discretionary contribution amount	$ 10,000,000
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		4.00%
Vesting percentage of employee deferrals		100.00%
Cost related to savings and stock investment plan		$ 3,201,000	$ 2,069,000	1,894,000
Employer contribution in shares of common stock		998,041	893,440
Dividends paid on United common stock		$ 1,218,000	$ 1,173,000	$ 1,139,000
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%
Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Amount of United common stock held in the pension plan		$ 24,003,000	$ 20,629,000
Percentage of equity securities include united common stock		3.00%	4.00%
Common Stock [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Maximum percentage limit of common stock to invest in portfolio for any industry			5.00%
Marketable Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Amount of United common stock held in the pension plan		$ 3,677,000	$ 4,894,000
Percentage of equity securities include united common stock		71.00%	71.00%
Maximum percentage limit of common stock to invest in portfolio for any industry		68.00%
Approximate fair value of plan assets		$ 34,682,000	$ 41,322,000
Marketable Equity Securities [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Maximum percentage limit of common stock to invest in portfolio for any industry		50.00%
Marketable 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%