Investments - Reconciliation for All Investments Measured at Fair Value on Recurring Basis Using Unobservable Inputs (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Level 3 investments, beginning of period	$ 185,612	$ 153,823	$ 141,221	$ 137,454
Purchases	7,084	2,196	57,246	12,760
Sales	(1,171)	(62)	(14,335)	(2,771)
Net realized and unrealized gains (losses) through earnings	560	(2,274)	7,953	6,240
Net transfers into and/or (out of) Level 3
Level 3 investments, end of period	192,085	153,683	192,085	153,683
Fixed maturity investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Level 3 investments, beginning of period	562	552	519	1,444
Sales				(1,043)
Net realized and unrealized gains (losses) through earnings	2	(42)	45	109
Net transfers into and/or (out of) Level 3
Level 3 investments, end of period	564	510	564	510
Other investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Level 3 investments, beginning of period	181,740	148,840	137,727	132,435
Purchases	7,084	2,196	57,246	12,760
Sales	(1,171)	(62)	(14,335)	(1,728)
Net realized and unrealized gains (losses) through earnings	576	(1,501)	7,591	6,006
Net transfers into and/or (out of) Level 3
Level 3 investments, end of period	188,229	149,473	188,229	149,473
Equity securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Level 3 investments, beginning of period	3,310	4,431	2,975	3,575
Net realized and unrealized gains (losses) through earnings	(18)	(731)	317	125
Net transfers into and/or (out of) Level 3
Level 3 investments, end of period	$ 3,292	$ 3,700	$ 3,292	$ 3,700