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Investments - Investment Income (Expense) Comprised Primarily of Interest and Dividends (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Net Investment Income [Line Items]    
Total investment income $ 8,752 $ 5,426
Less: Investment expenses (610) (641)
Net investment income 8,142 4,785
Debt Securities [Member]    
Net Investment Income [Line Items]    
Investment income 6,151 3,700
Equity Securities [Member]    
Net Investment Income [Line Items]    
Investment income 1,042 583
Short-term Investments [Member]    
Net Investment Income [Line Items]    
Investment income 0 89
Cash and Cash Equivalents [Member]    
Net Investment Income [Line Items]    
Investment income 1,300 858
Other [Member]    
Net Investment Income [Line Items]    
Investment income $ 259 $ 196