Financial instruments - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of Financial instruments	$ 26,050,000		$ 56,053,000
Transfers between Levels 1, 2 and 3	0		0
Short term investments consisting of available for sale securities	26,050,000		56,053,000
Net realized gains for securities	200,000	$ 200,000
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
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