Investments - Summary of Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Investment [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities	$ 6,766	$ 3,632
Trading Securities, Realized Gain (Loss)	2,664	344
Investment expense	(1,471)	(1,374)	$ (2,882)	$ (3,200)
Securities available-for-sale and short-term	78,329	61,742	124,881	172,293
Net investment income	86,459	66,662	141,301	176,902
Gains (losses) on securities held as of reporting date [Member]
Investment [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities			13,713	2,053
Trading Securities, Realized Gain (Loss)			3,572	1,325
Other Investments Income	4,102	3,288	10,141	728
Fixed Income Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	74,137	60,499	117,803	169,848
Short-Term [Member]
Investment [Line Items]
Gross Investment Income, Operating	5,481	2,438	9,594	5,278
Loans Receivable [Member]
Investment [Line Items]
Gross Investment Income, Operating	182	179	366	367
Other Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	$ 8,130	$ 4,920	$ 16,420	$ 4,609

Other Investments Income

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Line items represent reporting concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 12
-Sentence Column A

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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.

+ References

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=116637232&loc=SL114874131-224263

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+ 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)
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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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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
-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=116637232&loc=SL114874131-224263

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The sum of the realized and unrealized net gain (loss) on trading securities during the period.

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Amount of gain (loss) realized from securities classified as trading.

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