Acquisitions - Summary of Estimated Fair Values of Assets Acquired and Liabilities Assumed as of Date of Acquisition (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Nov. 13, 2015	May. 05, 2015	Jan. 27, 2015	Dec. 31, 2014	Apr. 01, 2014	Dec. 31, 2013	Nov. 25, 2013	Sep. 09, 2013	Mar. 31, 2013	Feb. 07, 2013
ASSETS
Prepaid reinsurance premiums	$ 121,427				$ 114,197
LIABILITIES
Goodwill	$ 73,071				$ 73,071		$ 60,071
Nationale Suisse Assurance S.A.
ASSETS
Short-term investments, trading, at fair value		$ 8,644
Short-term investments, available-for-sale, at fair value		6,687
Fixed maturities, trading, at fair value		31,350
Fixed maturities, available-for-sale, at fair value		96,656
Other investments		1,339
Total investments		144,676
Cash and cash equivalents		64,709
Reinsurance balances recoverable — reserves		4,343
Reinsurance balances recoverable — paids		12,151
Prepaid reinsurance premiums		3,213
Other assets		5,395
TOTAL ASSETS		234,487
LIABILITIES
Losses and LAE		173,209
Funds withheld		473
Insurance and reinsurance balances payable		6,991
Unearned premium		5,969
Other liabilities		12,620
TOTAL LIABILITIES		199,262
NET ASSETS ACQUIRED AT FAIR VALUE		35,225
Goodwill		0
Nationale Suisse Assurance S.A. | Life and Annuities Segment
ASSETS
Short-term investments, trading, at fair value		0
Short-term investments, available-for-sale, at fair value		6,687
Fixed maturities, trading, at fair value		0
Fixed maturities, available-for-sale, at fair value		96,656
Other investments		0
Total investments		103,343
Cash and cash equivalents		25,258
Reinsurance balances recoverable — reserves		302
Reinsurance balances recoverable — paids		1,320
Prepaid reinsurance premiums		0
Other assets		2,298
TOTAL ASSETS		132,521
LIABILITIES
Losses and LAE		117,188
Funds withheld		0
Insurance and reinsurance balances payable		779
Unearned premium		0
Other liabilities		2,875
TOTAL LIABILITIES		120,842
NET ASSETS ACQUIRED AT FAIR VALUE		11,679
Nationale Suisse Assurance S.A. | Non-life Run-off Segment
ASSETS
Short-term investments, trading, at fair value		8,644
Short-term investments, available-for-sale, at fair value		0
Fixed maturities, trading, at fair value		31,350
Fixed maturities, available-for-sale, at fair value		0
Other investments		1,339
Total investments		41,333
Cash and cash equivalents		39,451
Reinsurance balances recoverable — reserves		4,041
Reinsurance balances recoverable — paids		10,831
Prepaid reinsurance premiums		3,213
Other assets		3,097
TOTAL ASSETS		101,966
LIABILITIES
Losses and LAE		56,021
Funds withheld		473
Insurance and reinsurance balances payable		6,212
Unearned premium		5,969
Other liabilities		9,745
TOTAL LIABILITIES		78,420
NET ASSETS ACQUIRED AT FAIR VALUE		$ 23,546
StarStone
ASSETS
Short-term investments, trading, at fair value						$ 99,313
Fixed maturities, trading, at fair value						1,066,000
Other investments						2,068
Total investments						1,167,381
Cash and cash equivalents						339,608
Restricted cash and cash equivalents						22,779
Premiums receivable						321,350
Reinsurance balances recoverable — reserves						362,799
Reinsurance balances recoverable — paids						41,222
Prepaid reinsurance premiums						169,442
Intangible assets						43,900
Other assets						37,621
TOTAL ASSETS						2,506,102
LIABILITIES
Losses and LAE						1,264,246
Insurance and reinsurance balances payable						183,444
Unearned premium						392,962
Other liabilities						22,362
TOTAL LIABILITIES						1,863,014
NET ASSETS ACQUIRED AT FAIR VALUE						643,088
Goodwill						13,000
ACQUISITION DATE FAIR VALUE						656,088
StarStone | Non-life Run-off Segment
ASSETS
Short-term investments, trading, at fair value						25,888
Fixed maturities, trading, at fair value						329,235
Other investments						0
Total investments						355,123
Cash and cash equivalents						127,890
Restricted cash and cash equivalents						0
Premiums receivable						0
Reinsurance balances recoverable — reserves						152,057
Reinsurance balances recoverable — paids						20,100
Prepaid reinsurance premiums						25,221
Intangible assets						0
Other assets						0
TOTAL ASSETS						680,391
LIABILITIES
Losses and LAE						588,822
Insurance and reinsurance balances payable						42,447
Unearned premium						49,122
Other liabilities						0
TOTAL LIABILITIES						680,391
NET ASSETS ACQUIRED AT FAIR VALUE						0
Goodwill						0
ACQUISITION DATE FAIR VALUE						0
StarStone | StarStone Segment
ASSETS
Short-term investments, trading, at fair value						73,425
Fixed maturities, trading, at fair value						736,765
Other investments						2,068
Total investments						812,258
Cash and cash equivalents						211,718
Restricted cash and cash equivalents						22,779
Premiums receivable						321,350
Reinsurance balances recoverable — reserves						210,742
Reinsurance balances recoverable — paids						21,122
Prepaid reinsurance premiums						144,221
Intangible assets						43,900
Other assets						37,621
TOTAL ASSETS						1,825,711
LIABILITIES
Losses and LAE						675,424
Insurance and reinsurance balances payable						140,997
Unearned premium						343,840
Other liabilities						22,362
TOTAL LIABILITIES						1,182,623
NET ASSETS ACQUIRED AT FAIR VALUE						643,088
Goodwill						13,000
ACQUISITION DATE FAIR VALUE						$ 656,088
Wilton Relife Settlements
ASSETS
Other investments			$ 142,182
Cash and cash equivalents			5,043
Other assets			26,376
TOTAL ASSETS			173,601
LIABILITIES
TOTAL LIABILITIES			543
NET ASSETS ACQUIRED AT FAIR VALUE			173,058
Goodwill			$ 0
Sussex Insurance Company
ASSETS
Short-term investments, trading, at fair value				$ 85,309
Fixed maturities, trading, at fair value				523,227
Equities, trading, at fair value				31,439
Total investments				639,975
Cash and cash equivalents				358,458
Restricted cash and cash equivalents				15,279
Accrued interest receivable				3,984
Premiums receivable				37,190
Reinsurance balances recoverable				483,816
Prepaid reinsurance premiums				28,751
Other assets				43,939
TOTAL ASSETS				1,611,392
LIABILITIES
Losses and LAE				1,257,205
Funds withheld				42,090
Insurance and reinsurance balances payable				3,030
Unearned premium				79,293
Other liabilities				11,774
TOTAL LIABILITIES				1,393,392
NET ASSETS ACQUIRED AT FAIR VALUE				218,000
Goodwill				$ 0
Atrium
ASSETS
Short-term investments, trading, at fair value								$ 12,526
Short-term investments, available-for-sale, at fair value								33,535
Fixed maturities, trading, at fair value								10,751
Fixed maturities, available-for-sale, at fair value								156,142
Total investments								212,954
Cash and cash equivalents								44,842
Restricted cash and cash equivalents								12,305
Premiums receivable								41,855
Reinsurance balances recoverable								32,375
Deferred premium								26,224
Funds withheld								19,579
Intangible assets								90,000
Other assets								7,977
TOTAL ASSETS								488,111
LIABILITIES
Losses and LAE								216,319
Insurance and reinsurance balances payable								20,834
Unearned premium								42,738
Deferred taxes								39,740
Other liabilities								49,328
TOTAL LIABILITIES								368,959
NET ASSETS ACQUIRED AT FAIR VALUE								119,152
Goodwill								38,848
ACQUISITION DATE FAIR VALUE								$ 158,000
Arden
ASSETS
Short-term investments, trading, at fair value									$ 28,852
Fixed maturities, trading, at fair value									55,428
Other investments									2,867
Total investments									87,147
Cash and cash equivalents									23,037
Restricted cash and cash equivalents									31,812
Premiums receivable									124,221
Reinsurance balances recoverable									354,810
Other assets									12,016
TOTAL ASSETS									633,043
LIABILITIES
Losses and LAE									483,727
Insurance and reinsurance balances payable									59,304
Unearned premium									10,412
TOTAL LIABILITIES									553,443
NET ASSETS ACQUIRED AT FAIR VALUE									79,600
Arden | Non-life Run-off Segment
ASSETS
Short-term investments, trading, at fair value									12,512
Fixed maturities, trading, at fair value									46,077
Other investments									2,867
Total investments									61,456
Cash and cash equivalents									23,037
Restricted cash and cash equivalents									21,599
Premiums receivable									49,769
Reinsurance balances recoverable									354,810
Other assets									12,016
TOTAL ASSETS									522,687
LIABILITIES
Losses and LAE									427,567
Insurance and reinsurance balances payable									59,304
Unearned premium									10,412
TOTAL LIABILITIES									497,283
NET ASSETS ACQUIRED AT FAIR VALUE									25,404
Arden | Atrium Segment
ASSETS
Short-term investments, trading, at fair value									16,340
Fixed maturities, trading, at fair value									9,351
Other investments									0
Total investments									25,691
Cash and cash equivalents									0
Restricted cash and cash equivalents									10,213
Premiums receivable									74,452
Reinsurance balances recoverable									0
Other assets									0
TOTAL ASSETS									110,356
LIABILITIES
Losses and LAE									56,160
Insurance and reinsurance balances payable									0
Unearned premium									0
TOTAL LIABILITIES									56,160
NET ASSETS ACQUIRED AT FAIR VALUE									$ 54,196
Pavonia
ASSETS
Short-term investments, trading, at fair value										$ 40,404
Short-term investments, held-to-maturity, at fair value										10,268
Fixed maturities, trading, at fair value										329,985
Fixed maturities, held-to-maturity, at fair value										876,474
Total investments										1,257,131
Cash and cash equivalents										81,849
Accrued interest receivable										15,183
Funds held by reinsured companies										47,761
Other assets										59,002
TOTAL ASSETS										1,460,926
LIABILITIES
Policyholder benefits for life and annuity contracts										1,255,632
Reinsurance balances payable										39,477
Unearned premium										5,618
Other liabilities										4,635
TOTAL LIABILITIES										1,305,362
NET ASSETS ACQUIRED AT FAIR VALUE										$ 155,564
SeaBright
ASSETS
Short-term investments, trading, at fair value											$ 25,171
Fixed maturities, trading, at fair value											683,780
Total investments											708,951
Cash and cash equivalents											41,846
Accrued interest receivable											6,344
Premiums receivable											112,510
Reinsurance balances recoverable											117,462
Other assets											4,515
TOTAL ASSETS											991,628
LIABILITIES
Losses and LAE											592,774
Unearned premium											93,897
Loans payable											12,000
Insurance balances payable											3,243
Other liabilities											37,623
TOTAL LIABILITIES											739,537
NET ASSETS ACQUIRED AT FAIR VALUE											$ 252,091