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, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 5,641,645	$ 5,931,045
Gross Unrealized Gains	196,408	198,505
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	43,995	79,642
Estimated Fair Value	5,794,058	6,049,908
Non - credit other - than - temporary Impairments	16,481	35,232
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	2,785,608	2,622,566
Short-Term [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	716,666	430,827
Gross Unrealized Gains	10	5
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	160	44
Estimated Fair Value	716,516	430,788
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,825,555	5,435,385
Gross Unrealized Gains	196,398	198,428
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	43,835	79,598
Estimated Fair Value	4,978,118	5,554,215
Non - credit other - than - temporary Impairments	16,481	35,232
Fixed Income Securities [Member] \| Securities Pledged as Collateral [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	99,424	64,833
Gross Unrealized Gains	0	72
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Estimated Fair Value	99,424	64,905
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	537,068	376,064
Gross Unrealized Gains	10,827	5,509
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	8,641	7,205
Estimated Fair Value	539,254	374,368
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	1,278,995	1,803,136
Gross Unrealized Gains	12,804	19,589
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	6,331	20,560
Estimated Fair Value	1,285,468	1,802,165
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	29,863	41,932
Gross Unrealized Gains	510	1,303
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	163	100
Estimated Fair Value	30,210	43,135
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	90,960	33,732
Gross Unrealized Gains	519	2,551
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1,655	97
Estimated Fair Value	89,824	36,186
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		64,833
Gross Unrealized Gains		72
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments		0
Estimated Fair Value		64,905
Non - credit other - than - temporary Impairments		0
Fixed Income Securities [Member] \| U.S. Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0	4,063
Gross Unrealized Gains	0	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	3
Estimated Fair Value	0	4,060
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	2,197,085	2,284,425
Gross Unrealized Gains	101,487	110,955
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	18,933	43,785
Estimated Fair Value	2,279,639	2,351,595
Non - credit other - than - temporary Impairments	16,481	35,232
Fixed Income Securities [Member] \| Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	90,556	113,650
Gross Unrealized Gains	369	493
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	220
Estimated Fair Value	90,925	113,923
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	601,028	778,383
Gross Unrealized Gains	69,882	58,028
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	8,112	7,628
Estimated Fair Value	662,798	828,783
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member] \| Short-Term [Member] \| Securities Pledged as Collateral [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	99,424
Gross Unrealized Gains	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0
Estimated Fair Value	99,424
Non - credit other - than - temporary Impairments	0
Fixed Income Investments And Other Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,542,221	5,866,212
Gross Unrealized Gains	196,408	198,433
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	43,995	79,642
Estimated Fair Value	5,694,634	5,985,003
Non - credit other - than - temporary Impairments	$ 16,481	$ 35,232

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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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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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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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-Paragraph 2
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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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