Income taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Income Tax [Line Items]
Operating Loss Carry forwards Expiration Period											2024
Income Tax Expense (Benefit)	$ (7,389)	[1]	$ (652)	$ 5,314	$ 3,574	$ (1,071)	$ 2,933	$ 2,962	$ 1,804		$ 847	[2]	$ 6,628	[2]	$ 2,944
Operating Loss Carryforwards	5,400										$ 5,400
Operating Loss Carryforwards, Limitations on Use											All of the Companys remaining $4.1 million of NOLs are subject to annual limitations of $1.3 million.
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent											35.00%		35.00%		34.00%
Scenario, Plan [Member]
Income Tax [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent										21.00%
State and Local Jurisdiction [Member]
Income Tax [Line Items]
Operating Loss Carryforwards	$ 2,600										$ 2,600

[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.