Investment Securities (Non-agency Mortgage-backed Securities) (Details) (Fair Value, USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011		Sep. 30, 2011		Dec. 31, 2010
Investment securities
Fair value investments	$ 7,626.4	[1]	$ 8,346.8	[1],[2]	$ 7,265.9	[1]	$ 7,901.6	[1]
Residential Mortgage Backed Securities [Member]
Investment securities
Fair value investments	160.5		83.1
Commercial Mortgage Backed Securities [Member]
Investment securities
Fair value investments	389.3		276.7
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Investment securities
Fair value investments	549.8		359.8
Securities Issued In2007 [Member] | Residential Mortgage Backed Securities [Member]
Investment securities
Fair value investments	7.1
Securities Issued In2007 [Member] | Commercial Mortgage Backed Securities [Member]
Investment securities
Fair value investments	15.3
Securities Issued In2007 [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Investment securities
Fair value investments	22.4
Securities Issued In2009 [Member] | Residential Mortgage Backed Securities [Member]
Investment securities
Fair value investments	1.7
Securities Issued In2009 [Member] | Commercial Mortgage Backed Securities [Member]
Investment securities
Fair value investments	11.9
Securities Issued In2009 [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Investment securities
Fair value investments	13.6
Securities Issued in 2012 [Member] | Residential Mortgage Backed Securities [Member]
Investment securities
Fair value investments	0
Securities Issued in 2012 [Member] | Commercial Mortgage Backed Securities [Member]
Investment securities
Fair value investments	224.1
Securities Issued in 2012 [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Investment securities
Fair value investments	224.1
Securities Issued In2010 [Member] | Residential Mortgage Backed Securities [Member]
Investment securities
Fair value investments	53.2
Securities Issued In2010 [Member] | Commercial Mortgage Backed Securities [Member]
Investment securities
Fair value investments	6.1
Securities Issued In2010 [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Investment securities
Fair value investments	59.3
Securities Issued In2011 [Member] | Residential Mortgage Backed Securities [Member]
Investment securities
Fair value investments	74.9
Securities Issued In2011 [Member] | Commercial Mortgage Backed Securities [Member]
Investment securities
Fair value investments	127.4
Securities Issued In2011 [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Investment securities
Fair value investments	202.3
Securities Issued In2003 [Member] | Residential Mortgage Backed Securities [Member]
Investment securities
Fair value investments	2.2
Securities Issued In2003 [Member] | Commercial Mortgage Backed Securities [Member]
Investment securities
Fair value investments	0
Securities Issued In2003 [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Investment securities
Fair value investments	2.2
Securities Issued In 2006 [Member] | Residential Mortgage Backed Securities [Member]
Investment securities
Fair value investments	21.4
Securities Issued In 2006 [Member] | Commercial Mortgage Backed Securities [Member]
Investment securities
Fair value investments	4.5
Securities Issued In 2006 [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Investment securities
Fair value investments	$ 25.9

[1] (1) Excludes carrying value of $33.0 and $34.3 at January 1, 2012 and September 30, 2012 associated with other long-term investments accounted for using the equity method

[2] (2) Carrying value includes $111.8 and $377.3 at January 1, 2012 and September 30, 2012 that is classified as assets held for sale relating to discontinued operations. Level 3 Investments MillionsLevel 1InvestmentsLevel 2InvestmentsFixedMaturitiesCommonequitysecuritiesConvertiblefixedmaturitiesOther long-terminvestments Total Balance at January 1, 2011$1,894.4$5,477.4$128.4$71.2$—$330.2(1) $7,901.6(1) Total realized and unrealized gains (losses)(50.5)115.9(10.6)(4.9)—22.5 72.4 Foreign currency gains (losses) through OCI.3(84.4)(4.4)1.6—(5.0) (91.9) Amortization/Accretion2.4(40.7)(.1)——— (38.4) Purchases6,690.73,987.4212.819.7—27.9 10,938.5 Sales(6,782.1)(4,596.9)1.6(55.5)—(83.4) (11,516.3) Transfers in—111.81.0——— 112.8 Transfers out—(1.0)(111.8)——— (112.8) Balance at September 30, 2011$1,755.2$4,969.5$216.9$32.1$—$292.2(1) $7,265.9(1) (1) Excludes carrying value o