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, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 5,715,764	$ 5,931,045
Gross Unrealized Gains	208,763	198,505
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	68,565	79,642
Estimated Fair Value	5,855,962	6,049,908
Non - credit other - than - temporary Impairments	31,056	35,232
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	2,663,719	2,622,566
Short-Term [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	347,152	430,827
Gross Unrealized Gains	12	5
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	109	44
Estimated Fair Value	347,055	430,788
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,303,723	5,435,385
Gross Unrealized Gains	208,732	198,428
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	68,456	79,598
Estimated Fair Value	5,443,999	5,554,215
Non - credit other - than - temporary Impairments	31,056	35,232
Fixed Income Securities [Member] \| Securities Pledged as Collateral [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	64,889	64,833
Gross Unrealized Gains	19	72
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Estimated Fair Value	64,908	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	458,363	376,064
Gross Unrealized Gains	5,928	5,509
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	7,584	7,205
Estimated Fair Value	456,707	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	1,769,546	1,803,136
Gross Unrealized Gains	20,681	19,589
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	15,303	20,560
Estimated Fair Value	1,774,924	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	28,652	41,932
Gross Unrealized Gains	843	1,303
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	32	100
Estimated Fair Value	29,463	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	41,074	33,732
Gross Unrealized Gains	1,042	2,551
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	107	97
Estimated Fair Value	42,009	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	64,889	64,833
Gross Unrealized Gains	19	72
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Estimated Fair Value	64,908	64,905
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	4,020	4,063
Gross Unrealized Gains	0	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	2	3
Estimated Fair Value	4,018	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	2,240,984	2,284,425
Gross Unrealized Gains	110,762	110,955
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	37,259	43,785
Estimated Fair Value	2,314,487	2,351,595
Non - credit other - than - temporary Impairments	31,056	35,232
Fixed Income Securities [Member] \| Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	152,507	113,650
Gross Unrealized Gains	601	493
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	4	220
Estimated Fair Value	153,104	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	608,577	778,383
Gross Unrealized Gains	68,875	58,028
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	8,165	7,628
Estimated Fair Value	669,287	828,783
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	5,650,875	5,866,212
Gross Unrealized Gains	208,744	198,433
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	68,565	79,642
Estimated Fair Value	5,791,054	5,985,003
Non - credit other - than - temporary Impairments	$ 31,056	$ 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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-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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+ 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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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