Realized Gains and Losses, Excluding Net Other-Than-Temporary Impairment Losses Recognized in Earnings (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Net Realized Gains (Losses)	$ 37,141	$ 5,190	$ 1,694	$ 1,049	$ 4,043	$ 1,586	$ 1,960	$ 1,597	$ 45,073	$ 9,186	$ 8,373
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Gains									4,041	5,681	4,756
Losses									(3,249)	(4,271)	(5,926)
Net Realized Gains (Losses)									792	1,410	(1,170)
Short-Term Investments
Gain (Loss) on Investments [Line Items]
Gains									1,631	890	130
Losses									(612)	(1,552)	(383)
Net Realized Gains (Losses)									1,019	(662)	(253)
Equity Securities
Gain (Loss) on Investments [Line Items]
Gains									44,783	9,096	14,331
Losses									(1,521)	(658)	(4,535)
Net Realized Gains (Losses)									$ 43,262	$ 8,438	$ 9,796