Goodwill, Intangible Assets and Deferred Charge - Summary of Gross Carrying Value, Accumulated Amortization and Net Carrying Value of Intangible Assets (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value, Fair value adjustments	$ 368,610		$ 366,668
Accumulated Amortization, Fair value adjustments	(199,567)		(185,938)
Net Carrying Value, Fair value adjustments	169,043		180,730
Gross Carrying Value	42,000		42,000
Accumulated Amortization	(14,547)		(10,798)
Intangible assets with a definite life, Net Carrying Value	27,453		31,202
Indefinite-lived Intangible Assets [Line Items]
Intangible assets with an indefinite life, Carrying Value	87,031		87,031
Deferred charge on retroactive reinsurance, Gross Carrying Value	271,176		271,176
Deferred charge on retroactive reinsurance, Accumulated Amortization	(23,904)		(15,265)
Deferred charge on retroactive reinsurance, Net Carrying Value	247,272	$ 254,300	255,911	$ 265,426	$ 0	$ 0
Lloyd’s syndicate capacity
Indefinite-lived Intangible Assets [Line Items]
Intangible assets with an indefinite life, Carrying Value	37,031		37,031
Licenses
Indefinite-lived Intangible Assets [Line Items]
Intangible assets with an indefinite life, Carrying Value	19,900		19,900
Management contract
Indefinite-lived Intangible Assets [Line Items]
Intangible assets with an indefinite life, Carrying Value	30,100		30,100
Losses and LAE liabilities
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value, Fair value adjustments	458,202		456,110
Accumulated Amortization, Fair value adjustments	(320,714)		(307,785)
Net Carrying Value, Fair value adjustments	137,488		148,325
Reinsurance balances recoverable
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value, Fair value adjustments	(175,924)		(175,774)
Accumulated Amortization, Fair value adjustments	157,573		154,619
Net Carrying Value, Fair value adjustments	(18,351)		(21,155)
Policy benefits for life and annuity contracts
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value, Fair value adjustments	86,332		86,332
Accumulated Amortization, Fair value adjustments	(36,426)		(32,772)
Net Carrying Value, Fair value adjustments	49,906		53,560
Distribution channel
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	20,000		20,000
Accumulated Amortization	(3,445)		(2,777)
Intangible assets with a definite life, Net Carrying Value	16,555		17,223
Technology
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	15,000		15,000
Accumulated Amortization	(9,294)		(6,561)
Intangible assets with a definite life, Net Carrying Value	5,706		8,439
Brand
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	7,000		7,000
Accumulated Amortization	(1,808)		(1,460)
Intangible assets with a definite life, Net Carrying Value	$ 5,192		$ 5,540