Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Detail) - Successor [Member] - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 3,647,729		$ 5,271,818
Gross Unrealized Gains	241,436		145,386
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	21,618		108,043
Estimated Fair Value	3,867,547		5,309,161
Non - credit other - than - temporary Impairments	0		23,832
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	2,718,377		2,914,145
Short-Term [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	562,111		557,476
Gross Unrealized Gains	5		3
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	56		209
Estimated Fair Value	562,060		557,270
Non - credit other - than - temporary Impairments	0		0
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,001,432		4,614,623
Gross Unrealized Gains	241,431		145,383
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	21,562		107,834
Estimated Fair Value	3,221,301		4,652,172
Non - credit other - than - temporary Impairments	0		23,832
Fixed Income Securities [Member] \| Securities Pledged as Collateral [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	84,186		99,719
Gross Unrealized Gains	0		0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0		0
Estimated Fair Value	84,186		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	858,255		845,778
Gross Unrealized Gains	114,008		3,456
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1,799		69,400
Estimated Fair Value	970,464		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	1,324,241		858,774
Gross Unrealized Gains	12,077		6,772
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	16,950		5,471
Estimated Fair Value	1,319,368		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	34,045		26,245
Gross Unrealized Gains	220		409
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	258		111
Estimated Fair Value	34,007		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	93,574		86,900
Gross Unrealized Gains	413		261
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1,780		1,753
Estimated Fair Value	92,207		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	84,186		99,719
Gross Unrealized Gains	0		0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0		0
Estimated Fair Value	84,186		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	230,473		2,214,512
Gross Unrealized Gains	48,350		67,303
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	48		30,482
Estimated Fair Value	278,775		2,251,333
Non - credit other - than - temporary Impairments	0		23,832
Fixed Income Securities [Member] \| Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	87,642		50,754
Gross Unrealized Gains	83		283
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	4	$ 4	0
Estimated Fair Value	87,721		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	373,202		531,660
Gross Unrealized Gains	66,280		66,899
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	723		617
Estimated Fair Value	438,759		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	3,563,543		5,172,099
Gross Unrealized Gains	241,436		145,386
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	21,618		108,043
Estimated Fair Value	3,783,361		5,209,442
Non - credit other - than - temporary Impairments	$ 0		$ 23,832

Accumulated other comprehensive loss related to portion of other than temporary impairment (OTTI) due to factors other than credit losses on debt securities categorized as available-for-sale that an entity does not intend to sell and which it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis of the debt security. The amount represented by this item is net of tax, as applicable.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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