Investments - Summary of Proceeds Received and Gross Realized Gains and Losses from Sales of Available for Sale Fixed Maturity Securities (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt Securities, Available-for-sale [Line Items]
Proceeds			$ 78,540	$ 9,638
Fixed-Maturity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Proceeds	$ 771	$ 2,765	78,540	9,638
Gross Realized Gains	$ 71	97	1,232	126
Gross Realized Losses		$ (23)	$ (500)	$ (45)

Amount of cash inflow from sale of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26853-111562

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3151-108585

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3179-108585

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration