Premiums Written and Earned - Schedule of Net Premiums Written and Earned (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Effects of Reinsurance [Line Items]
Net, premiums written	$ 238,816	$ 211,314	$ 450,330	$ 413,068
Net Amount	226,928	212,023	436,337	410,929
Non-Life Run-Off
Effects of Reinsurance [Line Items]
Gross premiums written	7,066	14,797	13,763	24,914
Ceded, premiums written	(4,290)	(39,590)	(5,716)	(39,867)
Net, premiums written	2,776	(24,793)	8,047	(14,953)
Gross, premiums earned	9,216	53,184	17,163	78,157
Ceded, premiums earned	(4,740)	(35,886)	(7,252)	(42,367)
Net Amount	4,476	17,298	9,911	35,790
Atrium
Effects of Reinsurance [Line Items]
Gross premiums written	37,781	35,786	79,299	84,699
Ceded, premiums written	(4,619)	(3,966)	(7,957)	(8,521)
Net, premiums written	33,162	31,820	71,342	76,178
Gross, premiums earned	35,908	37,913	71,342	76,067
Ceded, premiums earned	(4,150)	(3,956)	(7,673)	(8,238)
Net Amount	31,758	33,957	63,669	67,829
StarStone
Effects of Reinsurance [Line Items]
Gross premiums written	223,368	241,057	440,412	431,754
Ceded, premiums written	(41,023)	(59,692)	(107,930)	(125,566)
Net, premiums written	182,345	181,365	332,482	306,188
Gross, premiums earned	208,548	195,963	402,664	364,495
Ceded, premiums earned	(37,513)	(58,267)	(77,547)	(103,177)
Net Amount	171,035	137,696	325,117	261,318
Life and Annuities
Effects of Reinsurance [Line Items]
Gross premiums written	20,533	22,922	38,459	45,655
Gross, premiums earned	$ 19,659	$ 23,072	$ 37,640	$ 45,992