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	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,958,298	$ 4,939,325
Gross Unrealized Gains	222,608	242,962
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	29,363	32,269
Estimated Fair Value	5,151,543	5,150,018
Non - credit other - than - temporary Impairments	9,844	7,773
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	2,704,657	2,743,050
Short-Term [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	220,980	360,069
Gross Unrealized Gains	0	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	4
Estimated Fair Value	220,980	360,065
Non - credit other - than - temporary Impairments	0	0
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,672,886	4,514,878
Gross Unrealized Gains	222,227	242,962
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	29,363	32,154
Estimated Fair Value	4,865,750	4,725,686
Non - credit other - than - temporary Impairments	9,844	7,773
Fixed Income Securities [Member] | Securities Pledged as Collateral [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	64,432	64,378
Gross Unrealized Gains	381	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	111
Estimated Fair Value	64,813	64,267
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	517,256	523,019
Gross Unrealized Gains	12,136	9,769
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	4,204	6,996
Estimated Fair Value	525,188	525,792
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	1,588,962	1,382,195
Gross Unrealized Gains	24,097	12,815
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	3,290	9,416
Estimated Fair Value	1,609,769	1,385,594
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	123,601	126,041
Gross Unrealized Gains	4,038	3,060
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	2,475	1,344
Estimated Fair Value	125,164	127,757
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	28,983	42,328
Gross Unrealized Gains	1,418	1,078
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	104	427
Estimated Fair Value	30,297	42,979
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	64,432	64,378
Gross Unrealized Gains	381	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	111
Estimated Fair Value	64,813	64,267
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	4,438	29,524
Gross Unrealized Gains	0	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	11	38
Estimated Fair Value	4,427	29,486
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	1,705,246	1,557,059
Gross Unrealized Gains	159,388	167,396
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	18,157	13,500
Estimated Fair Value	1,846,477	1,710,955
Non - credit other - than - temporary Impairments	9,844	7,773
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	88,569	21,346
Gross Unrealized Gains	102	50
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	433	274
Estimated Fair Value	88,238	21,122
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	615,831	833,366
Gross Unrealized Gains	21,048	48,794
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	689	159
Estimated Fair Value	636,190	882,001
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	4,893,866	4,874,947
Gross Unrealized Gains	222,227	242,962
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	29,363	32,158
Estimated Fair Value	5,086,730	5,085,751
Non - credit other - than - temporary Impairments	$ 9,844	$ 7,773