Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Detail) - Successor [Member] - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,119,358	$ 5,271,818
Gross Unrealized Gains	197,984	145,386
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	40,045	108,043
Estimated Fair Value	4,277,297	5,309,161
Non - credit other - than - temporary Impairments	4,963	23,832
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	2,955,763	2,914,145
Short-Term [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	321,210	557,476
Gross Unrealized Gains	4	3
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	95	209
Estimated Fair Value	321,119	557,270
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,713,908	4,614,623
Gross Unrealized Gains	197,980	145,383
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	39,950	107,834
Estimated Fair Value	3,871,938	4,652,172
Non - credit other - than - temporary Impairments	4,963	23,832
Fixed Income Securities [Member] \| Securities Pledged as Collateral [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	84,240	99,719
Gross Unrealized Gains	0	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Estimated Fair Value	84,240	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	853,001	845,778
Gross Unrealized Gains	14,307	3,456
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	11,921	69,400
Estimated Fair Value	855,387	779,834
Non - credit other - than - temporary Impairments	109	0
Fixed Income Securities [Member] \| Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,445,586	858,774
Gross Unrealized Gains	12,216	6,772
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	18,809	5,471
Estimated Fair Value	1,438,993	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	26,425	26,245
Gross Unrealized Gains	238	409
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	268	111
Estimated Fair Value	26,395	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	90,281	86,900
Gross Unrealized Gains	119	261
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	3,139	1,753
Estimated Fair Value	87,261	85,408
Non - credit other - than - temporary Impairments	0	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	84,240	99,719
Gross Unrealized Gains	0	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Estimated Fair Value	84,240	99,719
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member] \| U.S. Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Fixed Income Securities [Member] \| Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	726,998	2,214,512
Gross Unrealized Gains	101,218	67,303
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	4,920	30,482
Estimated Fair Value	823,296	2,251,333
Non - credit other - than - temporary Impairments	4,854	23,832
Fixed Income Securities [Member] \| Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	50,754	50,754
Gross Unrealized Gains	204	283
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Estimated Fair Value	50,958	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	520,863	531,660
Gross Unrealized Gains	69,678	66,899
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	893	617
Estimated Fair Value	589,648	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	4,035,118	5,172,099
Gross Unrealized Gains	197,984	145,386
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	40,045	108,043
Estimated Fair Value	4,193,057	5,209,442
Non - credit other - than - temporary Impairments	$ 4,963	$ 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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-Topic 320
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-Section 50
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-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
-Section S99
-Paragraph 1
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