Income Taxes - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Federal income tax expense applicable to sales and calls of securities	$ 1,178,000	$ 533,000	$ 156,000
Income taxes paid	60,431,000	49,114,000	27,057,000
State net operating loss carryforwards	0
Franchise & other taxes not on income	7,513,000	4,816,000	4,717,000
Estimate of unrecognized tax benefits, reasonable possible	United cannot reasonably estimate the amount of tax benefits it may recognize over the next 12 months
Accrued interest related to uncertain tax positions	569,000	455,000
Interest or penalties were recognized	$ 0	$ 0	$ 0