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, 2018	[1]	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	[1]	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Earnings Per Share Disclosure [Line Items]
Net (loss) income	$ (8,107)		$ (13,880)	$ (30,616)	$ (3,056)	$ (5,206)		$ 2,104	$ 8,525	$ 5,734	$ (55,659)		$ 11,157		$ 10,637
Weighted average number of shares - basic	42,600,898		42,530,476	42,403,140	42,403,029	42,216,987		41,869,528	41,567,848	41,507,941	42,484,972	[2]	41,764,230	[2]	34,104,476
Shares underlying options											0		1,002,613		1,312,434
Weighted average number of shares outstanding – diluted	42,600,898		42,530,476	42,403,140	42,403,029	42,216,987		43,463,982	43,550,226	43,503,889	42,484,972	[2]	42,766,843	[2]	35,416,910

[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.