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Investments - Investment Income (Expense) Comprised Primarily of Interest and Dividends (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Net Investment Income [Line Items]    
Total investment income $ 5,426 $ 3,251
Less: Investment expenses (641) (547)
Net investment (expense) income 4,785 2,704
Available-for-Sale Debt Securities [Member]    
Net Investment Income [Line Items]    
Investment income 3,700 2,710
Equity Securities [Member]    
Net Investment Income [Line Items]    
Investment income 583 382
Available-for-sale Short-term Investments [Member]    
Net Investment Income [Line Items]    
Investment income 89 15
Other [Member]    
Net Investment Income [Line Items]    
Investment income $ 1,054 $ 144