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Investments - Additional Information (Detail) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Investment Holdings [Line Items]        
Other-than-temporary impairment losses recognized for the period $ 0 $ 2,800,000 $ 0 $ 2,800,000