Loss and Loss Adjustment Expenses - Summary Exposures to Various Asbestos Related Claims (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Asbestos Claims [Line Items]
Balance, beginning of period	$ 718,033	$ 687,602	$ 614,557
Incurred losses and LAE during the current year	199,031	214,573	244,916
Loss and LAE payments	(52,085)	(53,955)	(52,848)
Balance, end of period	742,776	718,033	687,602
Asbestos Issue [Member]
Asbestos Claims [Line Items]
Balance, beginning of period	958	819	219
Incurred losses and LAE during the current year	816	444	882
Loss and LAE payments	(287)	(305)	(282)
Balance, end of period	$ 1,487	$ 958	$ 819