Realized Gains and Losses on Investment Sold, Excluding Net Other-Than-Temporary Impairment Losses Recognized in Earnings (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Gain (Loss) on Investments [Line Items]
Net Realized Gains (Losses) on Investments Sold	$ 253	$ 5,190	$ 3,210	$ 7,933
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Gains	331	2,589	3,678	3,401
Losses	(369)	(900)	(1,125)	(3,114)
Net Realized Gains (Losses) on Investments Sold	(38)	1,689	2,553	287
Short-Term Investments, Cash & Cash Equivalents
Gain (Loss) on Investments [Line Items]
Gains	107	758	189	984
Losses	(71)	(14)	(266)	(139)
Net Realized Gains (Losses) on Investments Sold	36	744	(77)	845
Other Invested Assets
Gain (Loss) on Investments [Line Items]
Gains	9	9	78	30
Losses	(50)	(97)	(136)	(232)
Net Realized Gains (Losses) on Investments Sold	(41)	(88)	(58)	(202)
Equity Securities
Gain (Loss) on Investments [Line Items]
Gains	347		1,102
Gains		2,863		7,249
Losses	(51)		(310)
Losses		(18)		(246)
Net Realized Gains (Losses) on Investments Sold	$ 296		$ 792
Net Realized Gains (Losses) on Investments Sold		$ 2,845		$ 7,003

Amount of gross realized gain from sale of marketable securities.

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Amount of gross realized loss from sale of marketable securities. Excludes other than temporary impairments (OTTI).

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This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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Amount of loss realized on the sale of debt or equity securities classified as available-for-sale. Excludes other than temporary impairments (OTTI).

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Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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Amount of realized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), excluding other-than-temporary impairment (OTTI).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of realized gain (loss) from sale of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
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Amount of realized gain (loss) on investment in marketable security, excluding other-than-temporary impairment (OTTI).

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