Investment Securities (Fair Value Measurement by Level) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Investment securities
Percentage of investments recorded at fair value		93.00%	95.00%
Maximum percentage of price difference provided by pricing services		5.00%
Maximum price difference provided by pricing services	$ 1.3	$ 1.0
Fair value investments		5,991.4	6,766.5	6,950.7
Fair value, investment-related liabilities		38.0	86.3
Debt securities issued by corporations
Investment securities
Fair value investments		2,311.2	2,347.2
Mortgage-backed and asset-backed securities
Investment securities
Fair value investments		1,840.9	2,014.5
Other long-term investments
Investment securities
Carrying value of investment accounted for using the equity method		23.2
Discontinued Operations
Investment securities
Investments reclassified to assets held for sale			236.3	338.1
Level 1
Investment securities
Fair value investments		843.3	1,376.7	1,355.1
Level 2
Investment securities
Fair value investments		4,638.0	4,982.2	5,206.1
Level 2 | Mortgage-backed and asset-backed securities
Investment securities
Fair value investments		1,840.9	1,992.5
Level 3 | Mortgage-backed and asset-backed securities
Investment securities
Fair value investments		0	22.0
Level 3 | Fixed maturity investments
Investment securities
Fair value investments		76.7	93.0	92.9
Level 3 | Common equity securities
Investment securities
Fair value investments		40.2	46.1	37.3
Level 3 | Convertible fixed maturity and preferred investments
Investment securities
Fair value investments		8.2	6.1	0
Level 3 | Other long-term investments
Investment securities
Fair value investments		385.0	262.4	259.3
Fair value measured on a recurring basis
Investment securities
Fair value investments		6,863.1	7,402.4
Fair value measured on a recurring basis | US Government and agency obligations
Investment securities
Fair value investments		188.1	362.5
Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fair value investments		2,311.2	2,347.2
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials
Investment securities
Fair value investments		456.6	434.4
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer
Investment securities
Fair value investments		777.6	754.4
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial
Investment securities
Fair value investments		285.0	281.1
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications
Investment securities
Fair value investments		237.5	265.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities
Investment securities
Fair value investments		169.8	173.6
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy
Investment securities
Fair value investments		181.8	159.7
Fair value measured on a recurring basis | Debt securities issued by corporations | Basic Materials
Investment securities
Fair value investments		108.6	149.1
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology
Investment securities
Fair value investments		87.9	91.2
Fair value measured on a recurring basis | Debt securities issued by corporations | Other
Investment securities
Fair value investments		6.4	38.7
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities
Investment securities
Fair value investments		1,840.9	2,014.5
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations
Investment securities
Fair value investments		275.1	439.9
Fair value measured on a recurring basis | Preferred stocks
Investment securities
Fair value investments		85.8	84.8
Fair value measured on a recurring basis | Municipal obligations
Investment securities
Fair value investments		83.2	17.9
Fair value measured on a recurring basis | Fixed maturity investments
Investment securities
Fair value investments		4,784.3	5,266.8
Fair value measured on a recurring basis | Short-term investments
Investment securities
Fair value investments		871.7	635.9		630.6
Fair value measured on a recurring basis | Common equity securities | Financials
Investment securities
Fair value investments		233.9	360.4
Fair value measured on a recurring basis | Common equity securities | Consumer
Investment securities
Fair value investments		238.4	308.2
Fair value measured on a recurring basis | Common equity securities | Industrial
Investment securities
Fair value investments		91.7	105.4
Fair value measured on a recurring basis | Common equity securities | Communications
Investment securities
Fair value investments		45.1	57.1
Fair value measured on a recurring basis | Common equity securities | Utilities
Investment securities
Fair value investments		9.5	34.3
Fair value measured on a recurring basis | Common equity securities | Energy
Investment securities
Fair value investments		32.7	78.6
Fair value measured on a recurring basis | Common equity securities | Basic Materials
Investment securities
Fair value investments		21.5	53.4
Fair value measured on a recurring basis | Common equity securities | Technology
Investment securities
Fair value investments		36.6	60.6
Fair value measured on a recurring basis | Common equity securities | Other
Investment securities
Fair value investments		92.2	98.8
Fair value measured on a recurring basis | Common equity securities | Common equity securities
Investment securities
Fair value investments		801.6	1,156.8
Fair value measured on a recurring basis | Convertible fixed maturity and preferred investments
Investment securities
Fair value investments		20.5	80.5
Fair value measured on a recurring basis | Other long-term investments
Investment securities
Fair value investments		385.0	262.4
Fair value measured on a recurring basis | Level 1
Investment securities
Fair value investments		1,712.1	1,998.2
Fair value measured on a recurring basis | Level 1 | US Government and agency obligations
Investment securities
Fair value investments		134.1	295.8
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Financials
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Consumer
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Industrial
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Communications
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Utilities
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Energy
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Basic Materials
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Technology
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Other
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 1 | Mortgage-backed and asset-backed securities
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 1 | Foreign government, agency and provincial obligations
Investment securities
Fair value investments		21.3	44.5
Fair value measured on a recurring basis | Level 1 | Preferred stocks
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 1 | Municipal obligations
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 1 | Fixed maturity investments
Investment securities
Fair value investments		155.4	340.3
Fair value measured on a recurring basis | Level 1 | Short-term investments
Investment securities
Fair value investments		868.8	621.5
Fair value measured on a recurring basis | Level 1 | Common equity securities | Financials
Investment securities
Fair value investments		193.7	314.3
Fair value measured on a recurring basis | Level 1 | Common equity securities | Consumer
Investment securities
Fair value investments		238.3	308.2
Fair value measured on a recurring basis | Level 1 | Common equity securities | Industrial
Investment securities
Fair value investments		91.7	105.4
Fair value measured on a recurring basis | Level 1 | Common equity securities | Communications
Investment securities
Fair value investments		45.1	57.1
Fair value measured on a recurring basis | Level 1 | Common equity securities | Utilities
Investment securities
Fair value investments		9.4	34.3
Fair value measured on a recurring basis | Level 1 | Common equity securities | Energy
Investment securities
Fair value investments		32.7	78.6
Fair value measured on a recurring basis | Level 1 | Common equity securities | Basic Materials
Investment securities
Fair value investments		21.5	53.4
Fair value measured on a recurring basis | Level 1 | Common equity securities | Technology
Investment securities
Fair value investments		36.6	60.6
Fair value measured on a recurring basis | Level 1 | Common equity securities | Other
Investment securities
Fair value investments		18.9	24.5
Fair value measured on a recurring basis | Level 1 | Common equity securities | Common equity securities
Investment securities
Fair value investments		687.9	1,036.4
Fair value measured on a recurring basis | Level 1 | Convertible fixed maturity and preferred investments
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 1 | Other long-term investments
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 2
Investment securities
Fair value investments		4,640.9	4,996.6
Fair value measured on a recurring basis | Level 2 | US Government and agency obligations
Investment securities
Fair value investments		54.0	66.7
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations
Investment securities
Fair value investments		2,305.6	2,347.2
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Financials
Investment securities
Fair value investments		456.6	434.4
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Consumer
Investment securities
Fair value investments		777.6	754.4
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Industrial
Investment securities
Fair value investments		285.0	281.1
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Communications
Investment securities
Fair value investments		237.5	265.0
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Utilities
Investment securities
Fair value investments		169.8	173.6
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Energy
Investment securities
Fair value investments		181.8	159.7
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Basic Materials
Investment securities
Fair value investments		103.0	149.1
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Technology
Investment securities
Fair value investments		87.9	91.2
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Other
Investment securities
Fair value investments		6.4	38.7
Fair value measured on a recurring basis | Level 2 | Mortgage-backed and asset-backed securities
Investment securities
Fair value investments		1,840.9	1,992.5
Fair value measured on a recurring basis | Level 2 | Foreign government, agency and provincial obligations
Investment securities
Fair value investments		253.8	395.4
Fair value measured on a recurring basis | Level 2 | Preferred stocks
Investment securities
Fair value investments		14.7	13.8
Fair value measured on a recurring basis | Level 2 | Municipal obligations
Investment securities
Fair value investments		83.2	17.9
Fair value measured on a recurring basis | Level 2 | Fixed maturity investments
Investment securities
Fair value investments		4,552.2	4,833.5
Fair value measured on a recurring basis | Level 2 | Short-term investments
Investment securities
Fair value investments		2.9	14.4
Fair value measured on a recurring basis | Level 2 | Common equity securities | Financials
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 2 | Common equity securities | Consumer
Investment securities
Fair value investments		0.1	0
Fair value measured on a recurring basis | Level 2 | Common equity securities | Industrial
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 2 | Common equity securities | Communications
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 2 | Common equity securities | Utilities
Investment securities
Fair value investments		0.1	0
Fair value measured on a recurring basis | Level 2 | Common equity securities | Energy
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 2 | Common equity securities | Basic Materials
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 2 | Common equity securities | Technology
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 2 | Common equity securities | Other
Investment securities
Fair value investments		73.3	74.3
Fair value measured on a recurring basis | Level 2 | Common equity securities | Common equity securities
Investment securities
Fair value investments		73.5	74.3
Fair value measured on a recurring basis | Level 2 | Convertible fixed maturity and preferred investments
Investment securities
Fair value investments		12.3	74.4
Fair value measured on a recurring basis | Level 2 | Other long-term investments
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 2 | Forward contracts
Investment securities
Change in fair value of derivatives, included in other revenue			(0.1)
Fair value measured on a recurring basis | Level 3
Investment securities
Fair value investments		510.1	407.6
Fair value measured on a recurring basis | Level 3 | US Government and agency obligations
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations
Investment securities
Fair value investments		5.6	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Financials
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Consumer
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Industrial
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Communications
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Utilities
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Energy
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Basic Materials
Investment securities
Fair value investments		5.6	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Technology
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Other
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 3 | Mortgage-backed and asset-backed securities
Investment securities
Fair value investments		0	22.0
Fair value measured on a recurring basis | Level 3 | Foreign government, agency and provincial obligations
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 3 | Preferred stocks
Investment securities
Fair value investments		71.1	71.0
Fair value measured on a recurring basis | Level 3 | Municipal obligations
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 3 | Fixed maturity investments
Investment securities
Fair value investments		76.7	93.0
Fair value measured on a recurring basis | Level 3 | Short-term investments
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 3 | Common equity securities | Financials
Investment securities
Fair value investments		40.2	46.1
Fair value measured on a recurring basis | Level 3 | Common equity securities | Consumer
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 3 | Common equity securities | Industrial
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 3 | Common equity securities | Communications
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 3 | Common equity securities | Utilities
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 3 | Common equity securities | Energy
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 3 | Common equity securities | Basic Materials
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 3 | Common equity securities | Technology
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 3 | Common equity securities | Other
Investment securities
Fair value investments		0	0
Fair value measured on a recurring basis | Level 3 | Common equity securities | Common equity securities
Investment securities
Fair value investments		40.2	46.1
Fair value measured on a recurring basis | Level 3 | Convertible fixed maturity and preferred investments
Investment securities
Fair value investments		8.2	6.1
Fair value measured on a recurring basis | Level 3 | Other long-term investments
Investment securities
Fair value investments		385.0	262.4
Carrying value of investment accounted for using the equity method		$ 23.2	$ 26.6	$ 35.0