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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Gain (Loss) on Investments [Line Items]
Net Realized Gains (Losses) on Investments Sold	$ (220)	$ 253	$ 1,787	$ 1,169					$ 2,989	$ 45,073	$ 9,186
Net Realized Gains (Losses) on Investments Sold					$ 37,141	$ 5,190	$ 1,694	$ 1,049
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Gains									3,775	4,041	5,681
Losses									(1,306)	(3,249)	(4,271)
Net Realized Gains (Losses) on Investments Sold									2,469	792	1,410
Short-Term Investments, Cash & Cash Equivalents
Gain (Loss) on Investments [Line Items]
Gains									208	1,575	871
Losses									(319)	(243)	(1,444)
Net Realized Gains (Losses) on Investments Sold									(111)	1,332	(573)
Other Invested Assets
Gain (Loss) on Investments [Line Items]
Gains									92	56	19
Losses									(213)	(369)	(108)
Net Realized Gains (Losses) on Investments Sold									(121)	(313)	(89)
Equity Securities
Gain (Loss) on Investments [Line Items]
Gains									1,102
Gains										44,783	9,096
Losses									(350)
Losses										(1,521)	(658)
Net Realized Gains (Losses) on Investments Sold									$ 752
Net Realized Gains (Losses) on Investments Sold										$ 43,262	$ 8,438