Loss and Loss Adjustment Expenses - Summary Exposures to Various Asbestos Related Claims (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Asbestos Claims [Line Items]
Balance, beginning of period	$ 771,845	$ 742,776	$ 718,033
Incurred losses and LAE during the current year	204,891	209,324	199,031
Loss and LAE payments	(62,061)	(56,951)	(52,085)
Balance, end of period	798,409	771,845	742,776
Asbestos Issue [Member]
Asbestos Claims [Line Items]
Balance, beginning of period	1,748	1,487	958
Incurred losses and LAE during the current year	108	556	816
Loss and LAE payments	(301)	(295)	(287)
Balance, end of period	$ 1,555	$ 1,748	$ 1,487