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, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 5,951,830	$ 5,283,157
Gross Unrealized Gains	222,697	155,169
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	63,773	104,206
Estimated Fair Value	6,110,754	5,334,120
Non - credit other - than - temporary Impairments	40,046	41,673
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	2,577,293	2,588,556
Short-Term [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	336,222	225,789
Gross Unrealized Gains	0	1
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	1
Estimated Fair Value	336,222	225,789
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,550,887	4,992,756
Gross Unrealized Gains	222,350	155,168
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	63,773	104,148
Estimated Fair Value	5,709,464	5,043,776
Non - credit other - than - temporary Impairments	40,046	41,673
Fixed Income Securities [Member] \| Securities Pledged as Collateral [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	64,721	64,612
Gross Unrealized Gains	347	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	57
Estimated Fair Value	65,068	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	367,363	424,048
Gross Unrealized Gains	12,402	4,910
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	4,113	8,188
Estimated Fair Value	375,652	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	1,681,035	1,610,912
Gross Unrealized Gains	50,637	7,089
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	969	24,332
Estimated Fair Value	1,730,703	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	69,374	96,638
Gross Unrealized Gains	6,017	1,491
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1	1,823
Estimated Fair Value	75,390	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	31,422	26,086
Gross Unrealized Gains	2,948	789
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1	188
Estimated Fair Value	34,369	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	64,721	64,612
Gross Unrealized Gains	347	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	57
Estimated Fair Value	65,068	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	4,152	4,239
Gross Unrealized Gains	0	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	4	27
Estimated Fair Value	4,148	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	2,221,276	1,942,285
Gross Unrealized Gains	103,989	99,670
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	54,722	64,617
Estimated Fair Value	2,270,543	1,977,338
Non - credit other - than - temporary Impairments	40,046	41,673
Fixed Income Securities [Member] \| Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	105,574	85,706
Gross Unrealized Gains	232	42
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1,244	1,481
Estimated Fair Value	104,562	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	1,070,691	802,842
Gross Unrealized Gains	46,125	41,177
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	2,719	3,492
Estimated Fair Value	1,114,097	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	5,887,109	5,218,545
Gross Unrealized Gains	222,350	155,169
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	63,773	104,149
Estimated Fair Value	6,045,686	5,269,565
Non - credit other - than - temporary Impairments	$ 40,046	$ 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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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=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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-URI http://asc.fasb.org/extlink&oid=27724398&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
-SubTopic 10
-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
-Section S99
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