Realized Gains and Losses, Excluding Net Other-Than-Temporary Impairment Losses Recognized in Earnings (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Gain (Loss) on Investments [Line Items]
Net Realized Gains (Losses)	$ 5,190	$ 1,586	$ 7,933	$ 5,143
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Gains	2,589	1,356	3,401	5,200
Losses	(900)	(1,221)	(3,114)	(3,086)
Net Realized Gains (Losses)	1,689	135	287	2,114
Short-Term Investments
Gain (Loss) on Investments [Line Items]
Gains	767	128	1,014	803
Losses	(111)	(107)	(371)	(250)
Net Realized Gains (Losses)	656	21	643	553
Equity Securities
Gain (Loss) on Investments [Line Items]
Gains	2,863	1,443	7,249	3,123
Losses	(18)	(13)	(246)	(647)
Net Realized Gains (Losses)	$ 2,845	$ 1,430	$ 7,003	$ 2,476