Loss and Loss Adjustment Expenses - Summary Exposures to Various Asbestos Related Claims (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Asbestos Claims [Line Items]
Balance, beginning of period	$ 772,887	$ 798,409	$ 771,845
Incurred losses and LAE during the current year	157,226	176,342	204,891
Loss and LAE payments	(50,113)	(58,883)	(62,061)
Balance, end of period	760,561	772,887	798,409
Asbestos Issue [Member]
Asbestos Claims [Line Items]
Balance, beginning of period	1,323	1,555	1,748
Incurred losses and LAE during the current year	(936)	(183)	108
Loss and LAE payments	(69)	(49)	(301)
Balance, end of period	$ 318	$ 1,323	$ 1,555