Fair Value Measurement (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 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 | RMBS
Fair Value Level 3 Rollforward
Balance at the beginning of the period	$ 222.4	$ 79.3	$ 112.4
Total pre-tax realized/unrealized gains/(losses) recorded in
Net income (loss)	(35.8)	5.2	(31.6)	5.6
Other comprehensive income (loss)	(15.6)	(41.4)	(46.5)	(59.6)
Purchases, issuances, sales, settlements, net		51.1		93.1
Purchases	1.7		152.2
Sales	(8.7)		(22.2)
Transfers in and/or out of Level 3		8.7		63.8
VIE Consolidations	(76.0)		(76.3)
Balance, at the end of the period	88.0	102.9	88.0	102.9
Total pre-tax realized/unrealized gains/(losses) recorded in
Other comprehensive income (loss)				59.6
Change in unrealized gains/(losses) related to financial instruments held at the reporting date	(15.6)	(41.4)	(46.5)	(59.6)
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	232.1	222.7	210.2	203.9
Total pre-tax realized/unrealized gains/(losses) recorded in
Net income (loss)	2.1	(14.6)	3.7	(15.2)
Other comprehensive income (loss)	(7.3)	8.5	13.0	9.1
Purchases, issuances, sales, settlements, net		13.7		13.7
Purchases	47.1		47.1
Sales	(0.1)		(0.1)
Balance, at the end of the period	273.9	230.3	273.9	230.3
Total pre-tax realized/unrealized gains/(losses) recorded in
Other comprehensive income (loss)			(13.0)	(9.1)
Change in unrealized gains/(losses) related to financial instruments held at the reporting date	(7.3)	8.5	13.0	9.1
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	2.2	4.4	2.3	0.2
Total pre-tax realized/unrealized gains/(losses) recorded in
Net income (loss)		(0.1)
Other comprehensive income (loss)	(0.2)	(0.2)	(0.3)	(0.2)
Purchases, issuances, sales, settlements, net		(1.5)		2.6
Balance, at the end of the period	2.0	2.6	2.0	2.6
Total pre-tax realized/unrealized gains/(losses) recorded in
Change in unrealized gains/(losses) related to financial instruments held at the reporting date	(0.2)		(0.3)
Level 3 | FG VIEs' assets, at fair value
Fair Value Level 3 Rollforward
Adoption of new accounting standard					1,925.3
Balance at the beginning of the period	3,679.0	1,868.6	3,657.5	1,925.3
Total pre-tax realized/unrealized gains/(losses) recorded in
Net income (loss)	(211.6)	(19.1)	22.8	(14.9)
Purchases, issuances, sales, settlements, net		(53.6)		(114.5)
Settlements	(257.6)		(470.5)
Consolidations, Deconsolidations, net		48.8		48.8
VIE Consolidations	282.4		282.4
Balance, at the end of the period	3,492.2	1,844.7	3,492.2	1,844.7	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	(84.7)	36.1	263.6	96.5
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	(2,142.2)	(1,284.9)	(1,872.6)	(1,542.1)
Total pre-tax realized/unrealized gains/(losses) recorded in
Net income (loss)	(59.4)	73.5	(295.1)	352.3
Purchases, issuances, sales, settlements, net		(63.5)		(85.0)
Settlements	17.3		16.6
Balance, at the end of the period	(2,184.3)	(1,274.9)	(2,184.3)	(1,274.9)
Change in unrealized gains/(losses) related to financial instruments held at the reporting date	(23.5)	36.7	(305.8)	294.6
Level 3 | FG VIEs' liabilities with recourse, at fair value
Fair Value Level 3 Rollforward
Adoption of new accounting standard					(2,110.9)
Balance, at the beginning of the period	(2,757.8)	(2,067.2)	(2,927.0)	(2,110.9)
Total pre-tax realized/unrealized gains/(losses) recorded in
Net income (loss)	(5.4)	21.9	(41.3)	12.3
Purchases, issuances, sales, settlements, net		67.5		120.8
Settlements	280.5		(485.6)
Consolidations, Deconsolidations, net		(71.5)		(71.5)
VIE Consolidations	(272.4)		(272.4)
Balance, at the end of the period	(2,755.1)	(2,049.3)	(2,755.1)	(2,049.3)	(2,110.9)
Change in unrealized gains/(losses) related to financial instruments held at the reporting date	7.1	(131.0)	43.0	(185.8)
Level 3 | FG VIEs' liabilities without recourse, at fair value
Fair Value Level 3 Rollforward
Adoption of new accounting standard					(226.0)
Balance, at the beginning of the period	(1,373.0)	(205.7)	(1,337.2)	(226.0)
Total pre-tax realized/unrealized gains/(losses) recorded in
Net income (loss)	(86.5)	(2.4)	(49.0)	(7.5)
Purchases, issuances, sales, settlements, net		23.2		48.6
Settlements	67.1		166.8
VIE Consolidations	(63.1)		(63.1)
Balance, at the end of the period	(1,282.5)	(184.9)	(1,282.5)	(184.9)	(226.0)
Change in unrealized gains/(losses) related to financial instruments held at the reporting date	$ (52.9)	$ 5.3	$ (119.1)	$ 1.9