Reinsurance (Details 2) (USD $) In Millions	Sep. 30, 2011
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Ceded Par Outstanding	$ 58,437
Second-to-Pay Insured Par Outstanding	27,427
Assumed Par Outstanding	44,782
Ceded par outstanding related to insured credit derivatives	5,994
Assumed Premium Receivable, net of Commissions	115.7
Assumed Expected Loss and LAE	(137.7)
Ceded Expected Loss and LAE	145.2
Radian Asset Assurance Inc.
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Ceded Par Outstanding	20,026
Second-to-Pay Insured Par Outstanding	56
Ceded Expected Loss and LAE	25.7
Tokio Marine & Nichido Fire Insurance Co., Ltd.
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Ceded Par Outstanding	17,117
Assumed Par Outstanding	933
Ceded Expected Loss and LAE	88.5
RAM Reinsurance Co. Ltd.
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Ceded Par Outstanding	12,067
Assumed Par Outstanding	24
Ceded Expected Loss and LAE	18.5
Syncora Guarantee Inc.
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Ceded Par Outstanding	4,203
Second-to-Pay Insured Par Outstanding	2,197
Assumed Par Outstanding	217
Assumed Expected Loss and LAE	(1.7)
Mitsui Sumitomo Insurance Co. Ltd.
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Ceded Par Outstanding	2,416
Ceded Expected Loss and LAE	7.1
ACA Financial Guaranty Corp.
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Ceded Par Outstanding	856
Second-to-Pay Insured Par Outstanding	12
Assumed Par Outstanding	2
Swiss Reinsurance Co.
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Ceded Par Outstanding	507
Ceded Expected Loss and LAE	2.4
Ambac
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Fixed maturity securities wrapped by monoline reinsurer	596.8
Ceded Par Outstanding	87
Second-to-Pay Insured Par Outstanding	7,551
Assumed Par Outstanding	23,259
Assumed Premium Receivable, net of Commissions	96.1
Assumed Expected Loss and LAE	(111.2)
CIFG Assurance North America Inc.
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Ceded Par Outstanding	69
Second-to-Pay Insured Par Outstanding	258
Assumed Par Outstanding	7,077
Assumed Premium Receivable, net of Commissions	6.8
Ceded Expected Loss and LAE	3.0
MBIA Insurance Corporation
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Fixed maturity securities wrapped by monoline reinsurer	803.9
Ceded Par Outstanding	40
Second-to-Pay Insured Par Outstanding	11,598
Assumed Par Outstanding	10,864
Assumed Premium Receivable, net of Commissions	0.2
Assumed Expected Loss and LAE	(16.4)
Financial Guaranty Insurance Co.
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Second-to-Pay Insured Par Outstanding	3,754
Assumed Premium Receivable, net of Commissions	12.6
Assumed Expected Loss and LAE	(8.4)
Other.
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Fixed maturity securities wrapped by monoline reinsurer	57.6
Ceded Par Outstanding	1,049
Second-to-Pay Insured Par Outstanding	2,001
Assumed Par Outstanding	$ 2,406