Fair Value Measurement (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Total pre-tax realized/unrealized gains/(losses) recorded in
Number of securities whose prices were determined to be reduced based on cash flow analysis by third party pricing services agency	1
Level 3 | Obligations of state and political subdivisions
Total pre-tax realized/unrealized gains/(losses) recorded in
Other comprehensive income (loss)	$ 0.4
Purchases	9.1
Balance at the end of the period	9.5
Total pre-tax realized/unrealized gains/(losses) recorded in
Change in unrealized gains/(losses) related to financial instruments held at the reporting date	0.4
Level 3 | RMBS
Fair Value Level 3 Rollforward
Balance at the beginning of the period	99.4
Total pre-tax realized/unrealized gains/(losses) recorded in
Net income (loss)	(23.3)	(0.1)
Other comprehensive income (loss)	(93.3)	(31.2)
Purchases, issuances, sales, settlements, net		90.3
Purchases	253.6
Sales	(4.1)
Settlements	(35.1)
Consolidations, Deconsolidations, net	(63.6)	(14.7)
Balance at the end of the period	133.6	99.4
Transfers in and/or out of Level 3		55.1
Total pre-tax realized/unrealized gains/(losses) recorded in
Change in unrealized gains/(losses) related to financial instruments held at the reporting date	(92.7)	(31.2)
Level 3 | Asset-backed securities
Fair Value Level 3 Rollforward
Balance before adjustment, at the beginning of period		203.9
Balance at the beginning of the period	210.2	203.9
Total pre-tax realized/unrealized gains/(losses) recorded in
Net income (loss)	(7.5)	(14.6)
Other comprehensive income (loss)	9.1	1.2
Purchases, issuances, sales, settlements, net		0.9
Purchases	47.1
Settlements	(23.4)
Balance at the end of the period	235.5	210.2
Transfers in and/or out of Level 3		18.8
Total pre-tax realized/unrealized gains/(losses) recorded in
Change in unrealized gains/(losses) related to financial instruments held at the reporting date	9.1	1.2
Level 3 | Other invested assets
Fair Value Level 3 Rollforward
Balance before adjustment, at the beginning of period		0.2
Balance at the beginning of the period	2.3	0.2
Total pre-tax realized/unrealized gains/(losses) recorded in
Other comprehensive income (loss)	(0.7)	(0.5)
Purchases, issuances, sales, settlements, net		2.6
Settlements	0.1
Balance at the end of the period	1.7	2.3
Total pre-tax realized/unrealized gains/(losses) recorded in
Change in unrealized gains/(losses) related to financial instruments held at the reporting date	(0.7)
Level 3 | FG VIEs' assets, at fair value
Fair Value Level 3 Rollforward
Adoption of new accounting standard		1,925.3	1,925.3
Balance at the beginning of the period	3,657.5	1,925.3
Total pre-tax realized/unrealized gains/(losses) recorded in
Net income (loss)	(314.3)	84.8
Other comprehensive income (loss)	0
Purchases, issuances, sales, settlements, net		(282.4)
Settlements	(806.5)
Consolidations, Deconsolidations, net	282.4	1,929.8
Balance at the end of the period	2,819.1	3,657.5	1,925.3
Total pre-tax realized/unrealized gains/(losses) recorded in
Change in unrealized gains/(losses) related to financial instruments held at the reporting date	160.9	243.2
Level 3 | Other assets
Total pre-tax realized/unrealized gains/(losses) recorded in
Net income (loss)	34.0
Balance at the end of the period	53.8
Transfers into Level 3	19.8
Total pre-tax realized/unrealized gains/(losses) recorded in
Change in unrealized gains/(losses) related to financial instruments held at the reporting date	34.0
Level 3 | Credit derivative asset (liability), net
Fair Value Level 3 Rollforward
Balance before adjustment, at the beginning of period		(1,542.1)
Balance at the beginning of the period		(1,542.1)
Fair Value Level 3 Rollforward
Balance at the beginning of the period	(1,869.9)
Total pre-tax realized/unrealized gains/(losses) recorded in
Net income (loss)	559.7	(1.6)
Purchases, issuances, sales, settlements, net		(326.2)
Settlements	6.3
Balance at the end of the period	(1,303.9)	(1,869.9)
Change in unrealized gains/(losses) related to financial instruments held at the reporting date	570.4	(120.9)
Level 3 | FG VIEs' liabilities with recourse, at fair value
Fair Value Level 3 Rollforward
Balance before adjustment, at the beginning of period		(2,110.9)
Adoption of new accounting standard		(2,110.9)
Balance at the beginning of the period	(3,030.9)
Total pre-tax realized/unrealized gains/(losses) recorded in
Net income (loss)	80.3	(45.4)
Purchases, issuances, sales, settlements, net		255.3
Settlements	826.1
Consolidations, Deconsolidations, net	(272.4)	(1,129.9)
Balance at the end of the period	(2,396.9)	(3,030.9)
Change in unrealized gains/(losses) related to financial instruments held at the reporting date	88.4	(323.8)
Level 3 | FG VIEs' liabilities without recourse, at fair value
Fair Value Level 3 Rollforward
Balance before adjustment, at the beginning of period		(226.0)
Adoption of new accounting standard		(226.0)
Balance at the beginning of the period	(1,337.2)
Total pre-tax realized/unrealized gains/(losses) recorded in
Net income (loss)	55.7	(35.4)
Purchases, issuances, sales, settlements, net		91.7
Settlements	283.1
Consolidations, Deconsolidations, net	(63.1)	(1,167.5)
Balance at the end of the period	(1,061.5)	(1,337.2)
Change in unrealized gains/(losses) related to financial instruments held at the reporting date	$ (78.3)	$ (7.5)