Components of Net Realized Investment Gains (Losses) (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Schedule Of Available For Sale Securities [Line Items]
Total net realized investment gains (losses)		$ (310)	$ 7,323	$ 7,342	$ (22,332)
Equity securities, Gross realized gains		1,629	4,942	8,577	14,669
Equity securities, Gross realized losses		(3,291)	(55)	(2,476)	(31,870)
Equity securities, Total net realized investment gains (losses)		(1,662)	4,887	6,101	(17,201)
Not Designated as Hedging Instrument | Interest Rate Swap
Schedule Of Available For Sale Securities [Line Items]
Gross realized gains		1,267	1,520	4,985	19,514
Gross realized losses		(1,362)	(1,360)	(4,485)	(41,673)
Total net realized investment gains (losses)	[1]	(95)	160	500	(22,159)
Fixed Income Securities
Schedule Of Available For Sale Securities [Line Items]
Gross realized gains		3,364	2,705	9,042	21,685
Gross realized losses		(1,917)	(429)	(8,301)	(4,657)
Total net realized investment gains (losses)		$ 1,447	$ 2,276	$ 741	$ 17,028

[1] Includes periodic net interest settlements related to the derivatives of $1.4 million for the quarters ended September 30, 2021 and 2020, respectively, and $4.2 million and $3.1 million for the nine months ended September 30, 2021 and 2020, respectively.