Premiums Written and Earned - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				2 Months Ended	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014 Non-Life Run-Off [Member]	Mar. 31, 2013 Non-Life Run-Off [Member]	Mar. 31, 2013 Non-Life Run-Off [Member] SeaBright [Member]	Mar. 31, 2014 Non-Life Run-Off [Member] SeaBright [Member]	Mar. 31, 2014 Active Underwriting [Member]	Mar. 31, 2013 Active Underwriting [Member]	Mar. 31, 2014 Active Underwriting [Member] Atrium [Member]	Mar. 31, 2013 Active Underwriting [Member] Atrium [Member]	Mar. 31, 2014 Life and Annuities [Member]	Mar. 31, 2013 Life and Annuities [Member]
Premiums Written And Earned [Line Items]
Net premiums written			$ 1,043	$ 9,708	$ 9,708	$ 1,000	$ 19,465		$ 19,465	$ 0
Net premiums earned	61,658	31,661	2,527	30,920	30,920	2,527	32,639		32,639	0	26,492	741
Net premiums written											25,996	741
Net premiums earned											$ 26,492	$ 741