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Fair Value Measurements - Summary of Information about Described Model Inputs Used to Determine Fair Value of Each Class of Credit Derivatives (Detail) - Successor [Member] $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) Transactions $ / shares	Dec. 31, 2017 USD ($) Transactions $ / shares
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative, Number of Instruments Held | Transactions	2	2
Other investments	$ 411,604	$ 431,630
Fair value of derivative liabilities	59,363
Other [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Notional outstanding	$ 303,650	$ 325,890
Weighted average reference obligation price | $ / shares	$ 98.3	$ 99.3
Weighted average life (WAL) in years	5 years 10 months 24 days	6 years 6 months
Weighted average credit rating	A	A
Weighted average relative change ratio	23.10%	23.60%
CVA percentage	6.75%	9.64%
Fair value of derivative liabilities	$ 1,175	$ 566
Real Estate [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments	33,030	33,154
Hedge Funds, Multi-strategy [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments	0	53,054
Interest Rate Contract [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments	179,856	136,603
Illiquid Investments [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments	$ 66,922	$ 67,787
Asset-backed Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs Coupon Rate	5.97%	5.97%
Fair Value Inputs Maturity	16 years 5 months 23 days	17 years 7 days
Fair Value Inputs Yield	12.00%	12.00%
Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value of derivative liabilities	$ 0
Variable Interest Entity, Primary Beneficiary [Member] | European Transactions [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs Coupon Rate	2.47%	0.40%
Fair Value Inputs Maturity	17 years 10 months 20 days	15 years 3 months 10 days
Fair Value Inputs Yield	3.35%	4.82%

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Fair value input weighted average credit rating.

+ References

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Fair value input weighted average obligation price.

+ References

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Fair value input weighted average relative change ratio.

+ References

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Fair value inputs coupon rate.

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Fair value inputs yield.

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- Definition

Credit valuation adjustment (CVA) as a percentage of the credit derivative liability calculated excluding the CVA.

+ References

No definition available.

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- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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Fair value, after offset of derivative asset, of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected not to be and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=d3e41678-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226006-175313

Reference 4: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 210
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-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226008-175313

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- Definition

The number of derivative instruments of a particular group held by the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5580258-113959

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Amount of investments classified as other.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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