Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Net Investment Income [Line Items]
Investment Income	$ 17,278	$ 16,401	$ 34,478	$ 33,805
Investment expenses	(683)	(753)	(1,630)	(1,547)
Net investment income	16,595	15,648	32,848	32,258
Fixed maturities
Net Investment Income [Line Items]
Investment Income	15,259	14,188	30,308	28,142
Equity Securities
Net Investment Income [Line Items]
Investment Income	1,873	1,971	3,840	5,204
Short-term investments
Net Investment Income [Line Items]
Investment Income	$ 146	$ 242	$ 330	$ 459