Summary of Significant Accounting Policies - Reconciliation of Shares Used to Determine Net Income per Share (Detail) - USD ($) $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2017	[1]	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Earnings Per Share Disclosure [Line Items]
Net income	$ (5,206)		$ 2,104	$ 8,525	$ 5,734	$ (1,926)	$ 4,789	$ 4,830	$ 2,944	$ 11,157	[2]	$ 10,637	[2]	$ 4,763
Weighted average number of shares - basic	42,216,987		41,869,528	41,567,848	41,507,941	36,527,250	33,873,479	33,128,518	32,888,659	41,764,230	[2]	34,104,476	[2]	32,546,840
Weighted average number of shares outstanding - diluted	42,216,987		43,463,982	43,550,226	43,503,889	36,527,250	35,297,585	34,270,337	33,944,876	42,766,843	[2]	35,416,910	[2]	33,936,099
Warrants
Earnings Per Share Disclosure [Line Items]
Shares underlying										0		0		300,846
Options
Earnings Per Share Disclosure [Line Items]
Shares underlying										1,002,613		1,312,434		1,088,413

[1] As discussed previously, the fourth quarter 2017 results include the results of ARI subsequent to the acquisition on October 10, 2017.

[2] The sum of the net earnings per share may not add up to the full year amount due to rounding and because the quarterly calculations are based on varying numbers of shares outstanding.