Investments - Schedule of Changes in Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	$ 639,822	$ 668,440
Included in realized gains / losses	0	0
Included in other comprehensive income	30,367	(15,125)
Acquisitions	0	20,300
Sales	0	0
Amortization	2,322	2,397
Other	(10,059)	(9,853)
Transfers into (out of) Level 3	0	0
Ending Balance	$ 662,452	$ 666,159
Percent of total fixed maturities	4.00%	4.00%
Asset- backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	$ 12,982	$ 14,049
Included in realized gains / losses	0	0
Included in other comprehensive income	441	(697)
Acquisitions	0	0
Sales	0	0
Amortization	0	0
Other	(220)	(199)
Transfers into (out of) Level 3	0	0
Ending Balance	$ 13,203	$ 13,153
Percent of total fixed maturities	0.00%	0.00%
Collateralized debt obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	$ 73,369	$ 71,581
Included in realized gains / losses	0	0
Included in other comprehensive income	6,306	5,183
Acquisitions	0	0
Sales	0	0
Amortization	2,315	2,389
Other	(2,912)	(2,957)
Transfers into (out of) Level 3	0	0
Ending Balance	$ 79,078	$ 76,196
Percent of total fixed maturities	1.00%	0.00%
Corporates
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	$ 553,471	$ 582,810
Included in realized gains / losses	0	0
Included in other comprehensive income	23,620	(19,611)
Acquisitions	0	20,300
Sales	0	0
Amortization	7	8
Other	(6,927)	(6,697)
Transfers into (out of) Level 3	0	0
Ending Balance	$ 570,171	$ 576,810
Percent of total fixed maturities	3.00%	4.00%

Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, amortization.

No definition available.

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- Definition

Other adjustments, including capitalized interest and foreign exchange.

+ References

No definition available.

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- Definition

Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Fixed Maturity And Equity Securities Percentage

+ References

No definition available.

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

Amount of gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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- Definition

Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy.

+ References

No definition available.

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- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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