Consolidation of Variable Interest Entities (Details) (USD $)	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2011 entity security segment agreement market subsidiary company policy D transaction counterparty	Jun. 30, 2011 entity	Sep. 30, 2010	Sep. 30, 2011 subsidiary statement ratingcategory security Y segment entity agreement market company policy D transaction counterparty	Sep. 30, 2010	Dec. 31, 2010 entity security policy
Consolidated VIEs
Total consolidated VIEs (in entities)		35
Number of VIEs whose debt was fully paid off	1
Total unpaid principal balance for the VIEs' assets that were over 90 days or more past due	$ 1,161,400,000			$ 1,161,400,000		$ 1,199,100,000
Change in the instrument specific credit risk of the VIEs' assets	(178,100,000)		152,300,000	(656,800,000)	47,200,000
Difference between the aggregate unpaid principal and aggregate fair value of the VIEs' liabilities	2,721,900,000			2,721,900,000		2,053,000,000
Consolidated FG VIEs
Number of VIEs required to be consolidated	34			34		29
Assets	3,005,380,000			3,005,380,000		3,657,481,000
Liabilities	3,708,600,000			3,708,600,000		4,368,100,000
Financial Guaranty Insurance Contracts
Total	3,966,500,000			3,966,500,000		3,630,500,000
Contractual Maturity
2012	8,000,000			8,000,000
2013	20,100,000			20,100,000
2014	166,400,000			166,400,000
Thereafter	3,772,000,000			3,772,000,000
Total	3,966,500,000			3,966,500,000		3,630,500,000
Net earned premiums	(19,400,000)		(12,800,000)	(56,800,000)	(34,400,000)
Net investment income	(4,000,000)			(4,700,000)
Net realized investment gains (losses)	6,700,000			7,200,000
Net change in fair value of financial guaranty variable interest entities	(99,230,000)		171,258,000	(153,915,000)	134,953,000
Loss and loss adjustment expenses	37,900,000		10,200,000	105,500,000	44,400,000
Total pre-tax effect on net income	(78,000,000)		168,700,000	(102,700,000)	145,000,000
Less: tax provision (benefit)	(27,300,000)		59,100,000	(35,900,000)	50,800,000
Total effect on net income	(50,700,000)		109,600,000	(66,800,000)	94,200,000
Total effect on shareholders' equity				(393,400,000)		(371,400,000)
HELOCs
Consolidated FG VIEs
Number of VIEs required to be consolidated	8			8		8
Assets	651,600,000			651,600,000		857,100,000
Liabilities	1,027,100,000			1,027,100,000		1,126,100,000
Alt-A first lien
Consolidated FG VIEs
Number of VIEs required to be consolidated	3			3
Assets	155,100,000			155,100,000
Liabilities	145,400,000			145,400,000
Subprime
Consolidated FG VIEs
Number of VIEs required to be consolidated	5			5		5
Assets	429,400,000			429,400,000		528,700,000
Liabilities	512,100,000			512,100,000		616,500,000
Option ARM
Consolidated FG VIEs
Number of VIEs required to be consolidated	2			2		2
Assets	556,800,000			556,800,000		626,600,000
Liabilities	766,700,000			766,700,000		909,400,000
Alt-A second liens
Consolidated FG VIEs
Number of VIEs required to be consolidated	10			10		5
Assets	639,900,000			639,900,000		747,400,000
Liabilities	684,700,000			684,700,000		818,400,000
Automobile loans
Consolidated FG VIEs
Number of VIEs required to be consolidated	5			5		7
Assets	278,500,000			278,500,000		486,800,000
Liabilities	278,500,000			278,500,000		486,800,000
Life insurance
Consolidated FG VIEs
Number of VIEs required to be consolidated	1			1		1
Assets	294,100,000			294,100,000		304,800,000
Liabilities	294,100,000			294,100,000		304,800,000
Credit card loans
Consolidated FG VIEs
Number of VIEs required to be consolidated						1
Assets						106,100,000
Liabilities						$ 106,100,000