Stock Based Compensation - Status of United's Nonvested Stock Option Awards (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Shares, Nonvested, Beginning balance	430,278
Shares, Granted	255,217	967,285
Shares, Vested	(174,624)
Shares, Forfeited or expired	(3,000)
Shares, Nonvested, Ending balance	507,871	430,278
Weighted-Average Grant Date Fair Value Per Share, Nonvested Beginning balance	$ 6.84
Weighted-Average Grant Date Fair Value Per Share, Granted	8.85
Weighted-Average Grant Date Fair Value Per Share, Vested	6.69
Weighted-Average Grant Date Fair Value Per Share, Forfeited or expired	8.85
Weighted-Average Grant Date Fair Value Per Share, Nonvested Ending balance	$ 7.89	$ 6.84