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Schedule of Total Investment Return (Detail) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Net Investment Income [Line Items]    
Net investment income $ 11,404 $ 8,644
Net realized investment gains (losses) (316) 775
Change in unrealized holding gains (losses) (17,623) 7,017
Net realized and unrealized investment returns (17,939) 7,792
Total investment return $ (6,535) $ 16,436
Total investment return % [1] (0.40%) 1.10%
Average investment portfolio [2] $ 1,538,651 $ 1,559,965
[1] Not annualized.
[2] Average of total cash and invested assets, net of receivable/payable for securities purchased and sold, as of the beginning and end of the period.