Stock Based Compensation - Additional Information (Detail) - USD ($)		12 Months Ended
	May 18, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Authorized shares of stock, option plan, maximum	1,700,000
Aggregate number of shares issued in respect of restricted stock awards	500,000
Options available for award each plan year	1,200,000
Maximum number of shares of restricted stock or shares subject to a restricted stock units award granted		50,000
Maximum number of options and SARs		100,000
Maximum number of stock options and SARs awarded		10,000
Maximum number of Shares of restricted stock or shares subject to a restricted stock units award granted to individual non-employee director		5,000
Vesting period of awards		1/3 per year
Number of Options granted		255,217	967,285
Recognition of compensation expense		$ 3,555,000	$ 2,817,000	$ 2,484,000
Number of share available for grant for prior plans		0
Maximum term for awards granted (years)		10 years
Weighted average risk free interest rate		2.13%
Weighted average dividend yield		3.00%
Weighted average volatility factors of the expected market price of United's common stock		0.262%
Weighted-average expected option life		6 years 11 months 23 days
Estimated fair value of the options at the date of grant		$ 8.85
Total unrecognized compensation cost related to nonvested option awards		$ 2,597,000
Weighted-average expense recognition period		1 year 2 months 12 days
Total fair value of awards vested		$ 1,168,000
Cash received from options exercised under the plans		$ 4,619,000	$ 13,337,000	$ 7,871,000
Shares issued related stock option exercises		220,905	519,482	259,454
Weighted-average grant-date fair value of options granted		$ 8.85	$ 6.97	$ 7.23
Total intrinsic value of options exercised		$ 3,721,000	$ 11,001,000	$ 2,380,000
Excess tax benefits from stock-based compensation arrangements		$ 2,201,000	$ 4,008,000	$ 1,023,000
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted		90,075	289,637
Total unrecognized compensation cost related to nonvested option awards		$ 4,297,000
Weighted-average expense recognition period		1 year 3 months 19 days
Weighted-average grant-date fair value of options granted		$ 45.21
Vesting period		4 years