Income taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended										12 Months Ended
	Dec. 31, 2018		Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Income Tax [Line Items]
Operating Loss Carry Forwards Expiration Period											2024
Income Tax Expense (Benefit)	$ 71	[1]	$ 9,447	$ (10,158)	$ (1,064)	$ (7,389)	[1]	$ (652)	$ 5,314	$ 3,574	$ (1,704)	[2]	$ 847	[2]	$ 6,628
Operating Loss Carryforwards	38,300										$ 38,300
Operating Loss Carryforwards, Limitations on Use											subject to annual limitations of 80% of earnings, $5.4 million expiring through 2023, which are subject to annual limitations of approximately $1.3 million
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent											21.00%		35.00%		35.00%
Deferred Tax Assets, Valuation Allowance	11,315					$ 0					$ 11,315		$ 0
Operating Loss Carry Forwards Expirations Date											2021
No Expiration [Member]
Income Tax [Line Items]
Operating Loss Carryforwards	32,900										$ 32,900
State and Local Jurisdiction [Member]
Income Tax [Line Items]
Operating Loss Carryforwards	$ 21,900										$ 21,900

[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.