Schedule of Total Investment Return (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Investments Debt And Equity Securities [Abstract]
Net investment income		$ 11,348	$ 11,750	$ 32,393	$ 34,108
Net realized investment gains (losses)		(2,690)	5,319	11,290	7,833
Change in unrealized holding gains and losses		10,021	(1,564)	53,457	(25,822)
Net realized and unrealized investment returns		7,331	3,755	64,747	(17,989)
Total investment return		$ 18,679	$ 15,505	$ 97,140	$ 16,119
Total investment return %	[1]	1.20%	1.00%	6.20%	1.10%
Average investment portfolio	[2]	$ 1,585,165	$ 1,541,975	$ 1,562,177	$ 1,533,825

[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.