Loss and Loss Adjustment Expenses - Summary Exposures to Various Asbestos Related Claims (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Asbestos Claims [Line Items]
Balance, beginning of period	$ 687,602	$ 614,557	$ 570,450
Incurred losses and LAE during the current year	214,573	244,916	228,973
Loss and LAE payments	(53,955)	(52,848)	(51,169)
Balance, end of period	718,033	687,602	614,557
Asbestos Issue [Member]
Asbestos Claims [Line Items]
Balance, beginning of period	819	219	167
Incurred losses and LAE during the current year	444	882	53
Loss and LAE payments	(305)	(282)	(1)
Balance, end of period	$ 958	$ 819	$ 219