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, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 6,002,458	$ 5,283,157
Gross Unrealized Gains	236,300	155,169
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	52,657	104,206
Estimated Fair Value	6,186,101	5,334,120
Non - credit other - than - temporary Impairments	35,594	41,673
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	2,828,685	2,588,556
Short-Term [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	130,732	225,789
Gross Unrealized Gains	0	1
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	1
Estimated Fair Value	130,732	225,789
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,806,949	4,992,756
Gross Unrealized Gains	236,105	155,168
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	52,657	104,148
Estimated Fair Value	5,990,397	5,043,776
Non - credit other - than - temporary Impairments	35,594	41,673
Fixed Income Securities [Member] \| Securities Pledged as Collateral [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	64,777	64,612
Gross Unrealized Gains	195	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	57
Estimated Fair Value	64,972	64,555
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	389,573	424,048
Gross Unrealized Gains	14,700	4,910
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	4,751	8,188
Estimated Fair Value	399,522	420,770
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	1,821,981	1,610,912
Gross Unrealized Gains	52,436	7,089
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1,661	24,332
Estimated Fair Value	1,872,756	1,593,669
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	53,431	96,638
Gross Unrealized Gains	3,498	1,491
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	6	1,823
Estimated Fair Value	56,923	96,306
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	30,638	26,086
Gross Unrealized Gains	2,810	789
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	23	188
Estimated Fair Value	33,425	26,687
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	64,777	64,612
Gross Unrealized Gains	195	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	57
Estimated Fair Value	64,972	64,555
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]
Amortized Cost	4,108	4,239
Gross Unrealized Gains	0	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	2	27
Estimated Fair Value	4,106	4,212
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	2,323,156	1,942,285
Gross Unrealized Gains	109,663	99,670
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	45,288	64,617
Estimated Fair Value	2,387,531	1,977,338
Non - credit other - than - temporary Impairments	35,594	41,673
Fixed Income Securities [Member] \| Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	106,582	85,706
Gross Unrealized Gains	521	42
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	259	1,481
Estimated Fair Value	106,844	84,267
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	1,077,480	802,842
Gross Unrealized Gains	52,477	41,177
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	667	3,492
Estimated Fair Value	1,129,290	840,527
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	5,937,681	5,218,545
Gross Unrealized Gains	236,105	155,169
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	52,657	104,149
Estimated Fair Value	6,121,129	5,269,565
Non - credit other - than - temporary Impairments	$ 35,594	$ 41,673

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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
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-URI http://asc.fasb.org/extlink&oid=49167202&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
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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=27724398&loc=d3e27161-111563

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+ References

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