Financial instruments - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Financial instruments	$ 72,590,000		$ 72,590,000		$ 120,804,000
Transfers between Levels 1, 2 and 3	0		0		0
Short term investments consisting of available for sale securities	72,590,000		72,590,000		70,758,000
Net realized gains for securities	300,000	$ 200,000	700,000	$ 400,000
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		0
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Financial instruments	$ 0		$ 0		$ 0