Reinsurance (Details 2) (USD $) In Millions	Jun. 30, 2011
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Ceded Par Outstanding	$ 60,933
Second-to-Pay Insured Par Outstanding	27,969
Assumed Par Outstanding	48,150
Ceded par outstanding related to insured credit derivatives	6,534
Assumed Premium Receivable, net of Commissions	133.0
Assumed Expected Loss and LAE	(151.9)
Ceded Expected Loss and LAE	76.0
Radian Asset Assurance Inc.
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Ceded Par Outstanding	20,859
Second-to-Pay Insured Par Outstanding	56
Ceded Expected Loss and LAE	13.7
Tokio Marine & Nichido Fire Insurance Co., Ltd.
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Ceded Par Outstanding	17,966
Assumed Par Outstanding	933
Ceded Expected Loss and LAE	47.3
RAM Reinsurance Co. Ltd.
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Ceded Par Outstanding	12,701
Assumed Par Outstanding	24
Ceded Expected Loss and LAE	9.1
Syncora Guarantee Inc.
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Ceded Par Outstanding	4,334
Second-to-Pay Insured Par Outstanding	2,323
Assumed Par Outstanding	217
Assumed Expected Loss and LAE	(0.3)
Ceded Expected Loss and LAE	0.1
Mitsui Sumitomo Insurance Co. Ltd.
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Ceded Par Outstanding	2,442
Ceded Expected Loss and LAE	2.9
ACA Financial Guaranty Corp.
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Ceded Par Outstanding	866
Second-to-Pay Insured Par Outstanding	13
Assumed Par Outstanding	2
Swiss Reinsurance Co.
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Ceded Par Outstanding	512
Ceded Expected Loss and LAE	1.6
Ambac
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Fixed maturity securities wrapped by monoline reinsurer	598.5
Ceded Par Outstanding	87
Second-to-Pay Insured Par Outstanding	7,700
Assumed Par Outstanding	23,593
Assumed Premium Receivable, net of Commissions	111.3
Assumed Expected Loss and LAE	(98.4)
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	10,158
Assumed Premium Receivable, net of Commissions	7.1
Assumed Expected Loss and LAE	(1.2)
Ceded Expected Loss and LAE	1.3
MBIA Insurance Corporation
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Fixed maturity securities wrapped by monoline reinsurer	828.8
Ceded Par Outstanding	45
Second-to-Pay Insured Par Outstanding	11,704
Assumed Par Outstanding	10,773
Assumed Premium Receivable, net of Commissions	1.2
Assumed Expected Loss and LAE	(21.0)
Financial Guaranty Insurance Co.
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Second-to-Pay Insured Par Outstanding	3,854
Assumed Par Outstanding	2,350
Assumed Premium Receivable, net of Commissions	13.4
Assumed Expected Loss and LAE	(31.0)
Other.
Exposure, Premium Receivable, and Loss and LAE by Reinsurer
Fixed maturity securities wrapped by monoline reinsurer	54.4
Ceded Par Outstanding	1,052
Second-to-Pay Insured Par Outstanding	2,061
Assumed Par Outstanding	$ 100