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Available-for-sale Securities and Fair Value on Financial Instruments - Additional Information (Detail) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Investments Debt And Equity Securities [Abstract]        
Other-than-temporary impairment losses $ 0 $ 0 $ 0 $ 0
Gross realized gains from the sale of available-for-sale investments $ 3,400,000   $ 3,500,000  
Gross realized losses from the sale or maturity of available-for-sale investments   $ 0   $ 0