Realized Gains and Losses on Investment Sold, Excluding Net Other-Than-Temporary Impairment Losses Recognized in Earnings (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Gain (Loss) on Investments [Line Items]
Net Realized Gains (Losses) on Investments Sold	$ 1,787	$ 1,694	$ 2,956	$ 2,743
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Gains	1,541	344	3,347	812
Losses	(481)	(958)	(756)	(2,214)
Net Realized Gains (Losses) on Investments Sold	1,060	(614)	2,591	(1,402)
Short-Term Investments, Cash & Cash Equivalents
Gain (Loss) on Investments [Line Items]
Gains	63	119	81	226
Losses	(21)	(45)	(195)	(125)
Net Realized Gains (Losses) on Investments Sold	42	74	(114)	101
Other Invested Assets
Gain (Loss) on Investments [Line Items]
Gains	20	16	70	21
Losses	(45)	(24)	(86)	(135)
Net Realized Gains (Losses) on Investments Sold	(25)	(8)	(16)	(114)
Equity Securities
Gain (Loss) on Investments [Line Items]
Gains	754		754
Gains		2,470		4,386
Losses	(44)		(259)
Losses		(228)		(228)
Net Realized Gains (Losses) on Investments Sold	$ 710		$ 495
Net Realized Gains (Losses) on Investments Sold		$ 2,242		$ 4,158