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Investments - Investment Income (Expense) Comprised Primarily of Interest and Dividends (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Net Investment Income [Line Items]        
Total investment income $ 6,467 $ 3,739 $ 11,893 $ 6,990
Less: Investment expenses (681) (516) (1,322) (1,063)
Net investment (expense) income 5,786 3,223 10,571 5,927
Available-for-Sale Debt Securities [Member]        
Net Investment Income [Line Items]        
Investment income 4,095 3,206 7,795 5,916
Equity Securities [Member]        
Net Investment Income [Line Items]        
Investment income 711 338 1,294 720
Available-for-sale Short-term Investments [Member]        
Net Investment Income [Line Items]        
Investment income 56 7 145 22
Other [Member]        
Net Investment Income [Line Items]        
Investment income $ 1,605 $ 188 $ 2,659 $ 332