Commitments and Contingencies (Detail) - Financial and Other Guarantee Exposures (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Opening number of financial guarantee contracts	4	37
Number of financial guarantee contracts matured, prepaid or commuted during the period	0	(33)
Closing number of financial guarantee contracts	4	4
Aggregate exposure outstanding	$ 115,464	$ 115,464
Weighted average contractual term to maturity	26.2 years	26.7 years
Principal [Member]
Principal outstanding	115,464	115,464
Interest [Member]
Interest outstanding	0	0
Legacy Financial Guarantee Business [Member]
Total gross claim liability recorded	1,399	1,399
Total unearned premiums and fees recorded	$ 389	$ 425