Losses and Loss Expense Reserve (Details 4) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012
Summary of rollforward of RMBS subrogation, by estimation approach
Discounted RMBS subrogation (gross of reinsurance), Beginning Balance	$ 2,720,266
Additional transactions reviewed, subrogation
Additional adverse sample loans reviewed, subrogation
Adverse loans repurchased by the sponsor, subrogation	36
All other changes, subrogation	49,889
Discounted RMBS subrogation (gross of reinsurance), Ending Balance	2,770,191
Adverse Samples [Member]
Summary of rollforward of RMBS subrogation, by estimation approach
Discounted RMBS subrogation (gross of reinsurance), Number of Transaction, Beginning Balance	30
Discounted RMBS subrogation (gross of reinsurance), Beginning Balance	1,457,472
Additional transactions reviewed, subrogation
Additional adverse sample loans reviewed, subrogation
Adverse loans repurchased by the sponsor, subrogation	36
All other changes, subrogation	95,687
All other changes, number of transaction	(1)
Discounted RMBS subrogation (gross of reinsurance), Ending Balance	1,553,195
Random Samples [Member]
Summary of rollforward of RMBS subrogation, by estimation approach
Discounted RMBS subrogation (gross of reinsurance), Number of Transaction, Beginning Balance	16
Discounted RMBS subrogation (gross of reinsurance), Beginning Balance	1,262,794
Additional transactions reviewed, subrogation
Additional adverse sample loans reviewed, subrogation
Adverse loans repurchased by the sponsor, subrogation
All other changes, subrogation	(45,798)
Discounted RMBS subrogation (gross of reinsurance), Ending Balance	$ 1,216,996