Premiums Written and Earned - Schedule of Net Premiums Written and Earned (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Premiums Written And Earned [Line Items]
Gross premiums written									$ 1,105,277
Net, premiums written									871,996	$ 649,954	$ 103,193
Ceded, premiums earned									(289,871)	(226,304)	(19,782)
Net Amount	$ 197,091	$ 231,051	$ 212,023	$ 198,906	$ 171,889	$ 195,987	$ 216,916	$ 61,658	839,071	646,450	239,807
Non-Life Run-Off
Premiums Written And Earned [Line Items]
Gross premiums written									38,704	12,818	14,166
Ceded, premiums written									(16,110)	(2,546)	(4,933)
Net, premiums written									22,594	10,272	9,233
Gross, premiums earned									116,494	45,684	124,262
Ceded, premiums earned									(72,125)	(14,516)	(11,651)
Net Amount									44,369	31,168	112,611
Atrium
Premiums Written And Earned [Line Items]
Gross premiums written									149,082	154,248	35,524
Ceded, premiums written									(14,502)	(17,973)	(5,420)
Net, premiums written									134,580	136,275	30,104
Gross, premiums earned									149,310	153,816	37,726
Ceded, premiums earned									(14,635)	(17,871)	(5,514)
Net Amount									134,675	135,945	32,212
StarStone Segment
Premiums Written And Earned [Line Items]
Gross premiums written									824,714	512,219	0
Ceded, premiums written									(196,287)	(113,045)	0
Net, premiums written									628,427	399,174	0
Gross, premiums earned									769,875	528,135	0
Ceded, premiums earned									(196,729)	(154,502)	0
Net Amount									573,146	373,633	0
Life and Annuities
Premiums Written And Earned [Line Items]
Life, premiums written									86,395	104,233	63,856
Ceded, premiums earned									(6,382)	(8,921)	(8,131)
Life, premiums earned									86,881	105,704	94,984
Net Amount									$ 86,881	$ 105,704	$ 94,984