Loss and Loss Adjustment Expenses - Summary Exposures to Various Asbestos Related Claims (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Asbestos Claims [Line Items]
Balance, beginning of period	$ 742,776	$ 718,033	$ 687,602
Incurred losses and LAE during the current year	209,324	199,031	214,573
Loss and LAE payments	(56,951)	(52,085)	(53,955)
Balance, end of period	771,845	742,776	718,033
Asbestos Issue [Member]
Asbestos Claims [Line Items]
Balance, beginning of period	1,487	958	819
Incurred losses and LAE during the current year	556	816	444
Loss and LAE payments	(295)	(287)	(305)
Balance, end of period	$ 1,748	$ 1,487	$ 958