Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Detail) - Successor [Member] - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 3,833,380	$ 5,271,818
Gross Unrealized Gains	194,408	145,386
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	34,773	108,043
Estimated Fair Value	3,993,015	5,309,161
Non - credit other - than - temporary Impairments	7,306	23,832
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	2,756,924	2,914,145
Short-Term [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	393,516	557,476
Gross Unrealized Gains	6	3
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	75	209
Estimated Fair Value	393,447	557,270
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,355,223	4,614,623
Gross Unrealized Gains	194,402	145,383
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	34,698	107,834
Estimated Fair Value	3,514,927	4,652,172
Non - credit other - than - temporary Impairments	7,306	23,832
Fixed Income Securities [Member] \| Securities Pledged as Collateral [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	84,641	99,719
Gross Unrealized Gains	0	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Estimated Fair Value	84,641	99,719
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member] \| Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	862,256	845,778
Gross Unrealized Gains	38,667	3,456
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1,801	69,400
Estimated Fair Value	899,122	779,834
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member] \| Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,354,633	858,774
Gross Unrealized Gains	15,508	6,772
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	19,354	5,471
Estimated Fair Value	1,350,787	860,075
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member] \| Foreign Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	25,347	26,245
Gross Unrealized Gains	269	409
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	174	111
Estimated Fair Value	25,442	26,543
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member] \| U.S. Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	83,579	86,900
Gross Unrealized Gains	629	261
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1,795	1,753
Estimated Fair Value	82,413	85,408
Non - credit other - than - temporary Impairments		0
Fixed Income Securities [Member] \| U.S. Government Obligations [Member] \| Securities Pledged as Collateral [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	84,641	99,719
Gross Unrealized Gains	0	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Estimated Fair Value	84,641	99,719
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member] \| Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	460,252	2,214,512
Gross Unrealized Gains	59,441	67,303
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	7,384	30,482
Estimated Fair Value	512,309	2,251,333
Non - credit other - than - temporary Impairments	7,306	23,832
Fixed Income Securities [Member] \| Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	65,255	50,754
Gross Unrealized Gains	202	283
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Estimated Fair Value	65,457	51,037
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member] \| Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	503,901	531,660
Gross Unrealized Gains	79,686	66,899
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	4,190	617
Estimated Fair Value	579,397	597,942
Non - credit other - than - temporary Impairments	0	0
Fixed Income Investments And Other Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,748,739	5,172,099
Gross Unrealized Gains	194,408	145,386
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	34,773	108,043
Estimated Fair Value	3,908,374	5,209,442
Non - credit other - than - temporary Impairments	$ 7,306	$ 23,832

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
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-Publisher FASB
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
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