Commitments and Contingencies (Details) - Financial and other guarantee exposures (USD $) In Thousands, unless otherwise specified		12 Months Ended
	Dec. 31, 2010 contracts	Dec. 31, 2012 Financial Guarantee Member contracts	Dec. 31, 2011 Financial Guarantee Member contracts	Dec. 31, 2012 Financial Guarantee Related to Securities Including Some Issued By European Financials [Member]	Dec. 31, 2012 Financial Guarantee Related to Future Scheduled Repayments On Government Subsidized Housing Project [Member]
Guarantor Obligations [Line Items]
Number of financial guarantee contracts at beginning of period	37	4	37
Number of financial guarantee contracts matured, prepaid or commuted during the period		0	(33)
Number of financial guarantee contracts at end of period	37	4	4
Principal outstanding	$ 900,000	$ 115,464	$ 115,464
Interest outstanding		0	0
Aggregate outstanding exposure		115,464	115,464	108,300	7,200
Total gross claim liability recorded		1,399	1,399
Total unearned premiums and fees recorded		$ 279	$ 425
Weighted average contractual term to maturity		25 years 4 months 24 days	26 years 8 months 12 days