Reinsurance Balances Recoverable - Reinsurance Balances Recoverable by Reinsurer (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 1,402,216	$ 1,474,004
Reinsurance Recoverables | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 1,402,216	$ 1,474,004
% of Total	100.00%	100.00%
Reinsurance Recoverables | Reinsurer Concentration Risk | Top Ten Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 875,517	$ 903,562
% of Total	62.40%	61.30%
Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Greater Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 515,215	$ 554,931
% of Total	36.80%	37.60%
Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Less Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 11,484	$ 15,511
% of Total	0.80%	1.10%
Non-Life Run-Off
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 1,037,039	$ 1,106,960
Non-Life Run-Off | Reinsurance Recoverables | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	1,037,039	1,106,960
Non-Life Run-Off | Reinsurance Recoverables | Reinsurer Concentration Risk | Top Ten Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	695,558	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	334,176	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	7,305	9,319
Atrium
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	26,902	26,282
Atrium | Reinsurance Recoverables | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	26,902	26,282
Atrium | Reinsurance Recoverables | Reinsurer Concentration Risk | Top Ten Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	21,671	21,394
Atrium | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Greater Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	4,468	4,253
Atrium | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Less Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	763	635
StarStone Segment
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	315,639	316,351
StarStone Segment | Reinsurance Recoverables | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	315,639	316,351
StarStone Segment | Reinsurance Recoverables | Reinsurer Concentration Risk | Top Ten Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	145,437	155,171
StarStone Segment | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Greater Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	168,201	158,417
StarStone Segment | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Less Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	2,001	2,763
Life and Annuities | Reinsurance Recoverables | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	22,636	24,411
Life and Annuities | Reinsurance Recoverables | Reinsurer Concentration Risk | Top Ten Reinsurers
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	12,851	13,254
Life and Annuities | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Greater Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	8,370	8,363
Life and Annuities | Reinsurance Recoverables | Reinsurer Concentration Risk | Other Reinsurers' Balances Less Than $1 Million
Ceded Credit Risk [Line Items]
Reinsurance Recoverables	$ 1,415	$ 2,794