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Financial instruments - Additional Information (Detail) - USD ($)
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Fair Value of Financial instruments $ 104,130,000   $ 104,130,000   $ 120,804,000
Transfers between Levels 1, 2 and 3 0   0   0
Short term investments consisting of available for sale securities 104,130,000   104,130,000   70,758,000
Net realized gains for securities 200,000 $ 100,000 400,000 $ 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   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