Investments - Investment Income (Expense) Comprised Primarily of Interest and Dividends (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net Investment Income [Line Items]
Total investment income	$ 7,423	$ 3,644	$ 19,316	$ 10,634
Less: Investment expenses	(781)	(559)	(2,103)	(1,622)
Net investment (expense) income	6,642	3,085	17,213	9,012
Available-for-Sale Debt Securities [Member]
Net Investment Income [Line Items]
Investment income	4,595	2,977	12,390	8,893
Equity Securities [Member]
Net Investment Income [Line Items]
Investment income	722	348	2,016	1,068
Available-for-sale Short-term Investments [Member]
Net Investment Income [Line Items]
Investment income			145	22
Other [Member]
Net Investment Income [Line Items]
Investment income	$ 2,106	$ 319	$ 4,765	$ 651

Amount, before investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.2)
-URI http://asc.fasb.org/extlink&oid=114873790&loc=SL114874131-224263

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Represents the total of interest and dividend income, including any amortization and accretion (as applicable) of discounts and premiums, earned from (1) loans and leases whether held-for-sale or held-in-portfolio; (2) investment securities; (3) federal funds sold; (4) securities purchased under agreements to resell; (5) investments in banker's acceptances, commercial paper, or certificates of deposit; (6) dividend income; or (7) other investments not otherwise specified herein.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.1-5)
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Amount of expenses related to the generation of investment income.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.9)
-URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.2)
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Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(2))
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