Reinsurance Balances Recoverable - Reinsurance Balances Recoverable by Reinsurer (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 1,474,004	$ 1,331,555
% of Total	100.00%
Reinsurance Recoverables | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 1,474,004	$ 1,331,555
% of Total	100.00%	100.00%
Reinsurance Recoverables | Reinsurer Concentration Risk | Top Ten Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 903,562	$ 864,166
% of Total	61.30%	64.90%
Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Greater Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 554,931	$ 453,734
% of Total	37.60%	34.10%
Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Less Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 15,511	$ 13,655
% of Total	1.10%	1.00%
Non-Life Run-Off
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 1,106,960	$ 930,459
% of Total	100.00%
Non-Life Run-Off | Reinsurance Recoverables | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 1,106,960	930,459
Non-Life Run-Off | Reinsurance Recoverables | Reinsurer Concentration Risk | Top Ten Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	713,743	667,325
Non-Life Run-Off | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Greater Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	383,898	256,929
Non-Life Run-Off | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Less Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	9,319	6,205
Atrium
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	26,282	29,567
Atrium | Reinsurance Recoverables | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	26,282	29,567
Atrium | Reinsurance Recoverables | Reinsurer Concentration Risk | Top Ten Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	21,394	23,635
Atrium | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Greater Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	4,253	4,917
Atrium | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Less Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	635	1,015
StarStone Segment
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 316,351	345,054
% of Total	100.00%
StarStone Segment | Reinsurance Recoverables | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 316,351	345,054
StarStone Segment | Reinsurance Recoverables | Reinsurer Concentration Risk | Top Ten Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	155,171	158,117
StarStone Segment | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Greater Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	158,417	181,196
StarStone Segment | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Less Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 2,763	5,741
Life and Annuities
Ceded Credit Risk [Line Items]
% of Total	100.00%
Life and Annuities | Reinsurance Recoverables | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 24,411	26,475
Life and Annuities | Reinsurance Recoverables | Reinsurer Concentration Risk | Top Ten Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	13,254	15,089
Life and Annuities | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Greater Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	8,363	10,692
Life and Annuities | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Less Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 2,794	$ 694