Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Detail) (Successor [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,939,325	$ 6,324,568
Gross Unrealized Gains	242,962	74,724
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	32,269	116,634
Estimated Fair Value	5,150,018	6,282,658
Non - credit other - than - temporary Impairments	7,773	852
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	2,743,050	2,475,182
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,557,059
Estimated Fair Value	1,710,955
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,346
Estimated Fair Value	21,122
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	833,366
Estimated Fair Value	882,001
Short-Term [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	360,069	271,118
Gross Unrealized Gains	0	1
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	4	0
Estimated Fair Value	360,065	271,119
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,514,878	5,927,254
Gross Unrealized Gains	242,962	74,696
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	32,154	116,634
Estimated Fair Value	4,725,686	5,885,316
Non - credit other - than - temporary Impairments	7,773	852
Fixed Income Securities [Member] | Securities Pledged as Collateral [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	64,378	126,196
Gross Unrealized Gains	0	27
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	111	0
Estimated Fair Value	64,267	126,223
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	523,019	1,405,293
Gross Unrealized Gains	9,769	857
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	6,996	28,427
Estimated Fair Value	525,792	1,377,723
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	1,382,195	1,508,377
Gross Unrealized Gains	12,815	4,886
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	9,416	23,894
Estimated Fair Value	1,385,594	1,489,369
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	126,041	131,709
Gross Unrealized Gains	3,060	69
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1,344	6,901
Estimated Fair Value	127,757	124,877
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	42,328	128,415
Gross Unrealized Gains	1,078	9
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	427	2,176
Estimated Fair Value	42,979	126,248
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	64,378	126,196
Gross Unrealized Gains	0	27
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	111	0
Estimated Fair Value	64,267	126,223
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	29,524	32,214
Gross Unrealized Gains	0	10
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	38	70
Estimated Fair Value	29,486	32,154
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	1,557,059	1,516,877
Gross Unrealized Gains	167,396	59,853
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	13,500	18,105
Estimated Fair Value	1,710,955	1,558,625
Non - credit other - than - temporary Impairments	7,773	852
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,346	184,118
Gross Unrealized Gains	50	217
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	274	463
Estimated Fair Value	21,122	183,872
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	833,366	1,020,251
Gross Unrealized Gains	48,794	8,795
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	159	36,598
Estimated Fair Value	882,001	992,448
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	4,874,947	6,198,372
Gross Unrealized Gains	242,962	74,697
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	32,158	116,634
Estimated Fair Value	5,085,751	6,156,435
Non - credit other - than - temporary Impairments	$ 7,773	$ 852