Premiums Written and Earned - Schedule of Net Premiums Written and Earned (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Premiums Written And Earned [Line Items]
Net, premiums written	$ 211,513	$ 205,461
Net Amount	209,409	198,906
Non-Life Run-Off
Premiums Written And Earned [Line Items]
Gross premiums written	6,697	10,785
Ceded, premiums written	(1,426)	2,762
Net, premiums written	5,271	13,547
Gross, premiums earned	7,947	27,755
Ceded, premiums earned	(2,512)	(9,263)
Net Amount	5,435	18,492
Atrium
Premiums Written And Earned [Line Items]
Gross premiums written	41,518	48,913
Ceded, premiums written	(3,338)	(4,555)
Net, premiums written	38,180	44,358
Gross, premiums earned	35,434	38,153
Ceded, premiums earned	(3,523)	(4,281)
Net Amount	31,911	33,872
StarStone Segment
Premiums Written And Earned [Line Items]
Gross premiums written	217,043	190,697
Ceded, premiums written	(66,907)	(65,874)
Net, premiums written	150,136	124,823
Gross, premiums earned	194,116	168,532
Ceded, premiums earned	(40,034)	(44,910)
Net Amount	154,082	123,622
Life and Annuities
Premiums Written And Earned [Line Items]
Life, premiums written	17,926	22,733
Life, premiums earned	$ 17,981	$ 22,920