Financial instruments - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Financial instruments	$ 56,053,000	$ 120,804,000
Short term investments consisting of available for sale securities	56,053,000	70,758,000
Net realized gains for securities	900,000	600,000	$ 100,000
Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Financial instruments	$ 0	$ 0