Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Detail) - Successor [Member] - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 5,931,045	$ 5,283,157
Gross Unrealized Gains	198,505	155,169
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	79,642	104,206
Estimated Fair Value	6,049,908	5,334,120
Non - credit other - than - temporary Impairments	35,232	41,673
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	2,622,566	2,588,556
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,284,425
Estimated Fair Value	2,351,595
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	113,650
Estimated Fair Value	113,923
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	778,383
Estimated Fair Value	828,783
Short-Term [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	430,827	225,789
Gross Unrealized Gains	5	1
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	44	1
Estimated Fair Value	430,788	225,789
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,435,385	4,992,756
Gross Unrealized Gains	198,428	155,168
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	79,598	104,148
Estimated Fair Value	5,554,215	5,043,776
Non - credit other - than - temporary Impairments	35,232	41,673
Fixed Income Securities [Member] \| Securities Pledged as Collateral [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	64,833	64,612
Gross Unrealized Gains	72	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	57
Estimated Fair Value	64,905	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	376,064	424,048
Gross Unrealized Gains	5,509	4,910
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	7,205	8,188
Estimated Fair Value	374,368	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	1,803,136	1,610,912
Gross Unrealized Gains	19,589	7,089
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	20,560	24,332
Estimated Fair Value	1,802,165	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	41,932	96,638
Gross Unrealized Gains	1,303	1,491
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	100	1,823
Estimated Fair Value	43,135	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	33,732	26,086
Gross Unrealized Gains	2,551	789
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	97	188
Estimated Fair Value	36,186	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	64,833	64,612
Gross Unrealized Gains	72	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	57
Estimated Fair Value	64,905	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	4,063	4,239
Gross Unrealized Gains	0	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	3	27
Estimated Fair Value	4,060	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	2,284,425	1,942,285
Gross Unrealized Gains	110,955	99,670
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	43,785	64,617
Estimated Fair Value	2,351,595	1,977,338
Non - credit other - than - temporary Impairments	35,232	41,673
Fixed Income Securities [Member] \| Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	113,650	85,706
Gross Unrealized Gains	493	42
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	220	1,481
Estimated Fair Value	113,923	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	778,383	802,842
Gross Unrealized Gains	58,028	41,177
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	7,628	3,492
Estimated Fair Value	828,783	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	5,866,212	5,218,545
Gross Unrealized Gains	198,433	155,169
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	79,642	104,149
Estimated Fair Value	5,985,003	5,269,565
Non - credit other - than - temporary Impairments	$ 35,232	$ 41,673

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- Definition

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
-Name Accounting Standards Codification
-Topic 320
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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
-Subparagraph (b)
-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
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-Section 50
-Paragraph 2
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-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
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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