Reinsurance Balances Recoverable - Reinsurance Balances Recoverable by Reinsurer (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 1,345,115	$ 1,474,004
Reinsurance Recoverables | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 1,345,115	$ 1,474,004
% of Total	100.00%	100.00%
Reinsurance Recoverables | Reinsurer Concentration Risk | Top Ten Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 842,075	$ 903,562
% of Total	62.60%	61.30%
Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Greater Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 486,551	$ 554,931
% of Total	36.20%	37.60%
Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Less Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 16,489	$ 15,511
% of Total	1.20%	1.10%
Non-Life Run-Off
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 990,179	$ 1,106,960
Non-Life Run-Off | Reinsurance Recoverables | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	990,179	1,106,960
Non-Life Run-Off | Reinsurance Recoverables | Reinsurer Concentration Risk | Top Ten Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	661,380	713,743
Non-Life Run-Off | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Greater Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	317,169	383,898
Non-Life Run-Off | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Less Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	11,630	9,319
Atrium
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	27,618	26,282
Atrium | Reinsurance Recoverables | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	27,618	26,282
Atrium | Reinsurance Recoverables | Reinsurer Concentration Risk | Top Ten Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	22,369	21,394
Atrium | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Greater Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	4,508	4,253
Atrium | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Less Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	741	635
StarStone
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	305,086	316,351
StarStone | Reinsurance Recoverables | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	305,086	316,351
StarStone | Reinsurance Recoverables | Reinsurer Concentration Risk | Top Ten Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	146,115	155,171
StarStone | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Greater Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	156,576	158,417
StarStone | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Less Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	2,395	2,763
Life and Annuities
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	22,232	24,411
Life and Annuities | Reinsurance Recoverables | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	22,232	24,411
Life and Annuities | Reinsurance Recoverables | Reinsurer Concentration Risk | Top Ten Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	12,211	13,254
Life and Annuities | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Greater Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	8,298	8,363
Life and Annuities | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Less Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 1,723	$ 2,794