Investment Securities (Details 5) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Investment securities
Carrying value	$ 6,221.9	$ 5,786.1
Total fixed maturity investments including assets held for sale	6,333.7
Fair Value
Investment securities
Fair value investments	8,346.8	7,901.6
Level 1 Inputs
Investment securities
Fair value investments	1,879.1	1,894.4
Level 2 Inputs
Investment securities
Fair value investments	6,088.2	5,477.4
Fixed maturity investments. | Level 3 Inputs
Investment securities
Fair value investments	78.9	128.4
U.S. Government and agency obligations
Investment securities
Carrying value	305.0	398.7
Debt securities issued by corporations
Investment securities
Fair value investments	2,135.1	2,178.0
Carrying value	2,135.1	2,178.1
Municipal obligations
Investment securities
Carrying value	2.7	3.3
Mortgage-backed and asset-backed securities
Investment securities
Carrying value	3,222.9	2,071.7
Mortgage-backed and asset-backed securities | Fair Value
Investment securities
Fair value investments	3,222.9	2,071.7
Mortgage-backed and asset-backed securities | Level 2 Inputs
Investment securities
Fair value investments	3,207.8	2,014.7
Mortgage-backed and asset-backed securities | Level 3 Inputs
Investment securities
Fair value investments	15.1	57.0
Non-cumulative perpetual preference shares
Investment securities
Carrying value	78.8	87.9
Common equity securities | Level 3 Inputs
Investment securities
Fair value investments	32.3	71.2
Other long-term investments | Level 3 Inputs
Investment securities
Fair value investments	268.3	330.2
Carrying value of investment accounted for using the equity method	33.0	41.9
Fair value measured on a recurring basis | Fair Value
Investment securities
Fair value investments	8,346.8	7,901.6
Fair value measured on a recurring basis | Level 1 Inputs
Investment securities
Fair value investments	1,879.1	1,894.4
Fair value measured on a recurring basis | Level 2 Inputs
Investment securities
Fair value investments	6,088.2	5,477.4
Fair value measured on a recurring basis | Level 3 Inputs
Investment securities
Fair value investments	379.5	529.8
Fair value measured on a recurring basis | Fixed maturity investments. | Fair Value
Investment securities
Fair value investments	6,333.7	5,786.1
Fair value measured on a recurring basis | Fixed maturity investments. | Fair Value | AutoOne
Investment securities
Fair value investments	111.8
Fair value measured on a recurring basis | Fixed maturity investments. | Level 1 Inputs
Investment securities
Fair value investments	365.7	477.8
Fair value measured on a recurring basis | Fixed maturity investments. | Level 2 Inputs
Investment securities
Fair value investments	5,889.1	5,179.9
Fair value measured on a recurring basis | Fixed maturity investments. | Level 3 Inputs
Investment securities
Fair value investments	78.9	128.4
Fair value measured on a recurring basis | U.S. Government and agency obligations | Fair Value
Investment securities
Fair value investments	305.0	398.7
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 1 Inputs
Investment securities
Fair value investments	296.2	388.9
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 2 Inputs
Investment securities
Fair value investments	8.8	9.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Fair Value
Investment securities
Fair value investments	2,135.1	2,178.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 Inputs
Investment securities
Fair value investments	3.8	6.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 Inputs
Investment securities
Fair value investments	2,131.3	2,171.7
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Fair Value
Investment securities
Fair value investments	790.7	769.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 2 Inputs
Investment securities
Fair value investments	790.7	769.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Fair Value
Investment securities
Fair value investments	359.4	511.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 2 Inputs
Investment securities
Fair value investments	359.4	511.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Fair Value
Investment securities
Fair value investments	239.6	265.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 1 Inputs
Investment securities
Fair value investments	3.8	6.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 2 Inputs
Investment securities
Fair value investments	235.8	259.5
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications | Fair Value
Investment securities
Fair value investments	225.8	226.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications | Level 2 Inputs
Investment securities
Fair value investments	225.8	226.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Basic materials | Fair Value
Investment securities
Fair value investments	195.7	125.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Basic materials | Level 2 Inputs
Investment securities
Fair value investments	195.7	125.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Fair Value
Investment securities
Fair value investments	140.1	106.7
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 2 Inputs
Investment securities
Fair value investments	140.1	106.7
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Fair Value
Investment securities
Fair value investments	155.8	151.5
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 2 Inputs
Investment securities
Fair value investments	155.8	151.5
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Fair Value
Investment securities
Fair value investments	24.5	22.1
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 2 Inputs
Investment securities
Fair value investments	24.5	22.1
Fair value measured on a recurring basis | Debt securities issued by corporations | Diversified | Fair Value
Investment securities
Fair value investments	3.5
Fair value measured on a recurring basis | Debt securities issued by corporations | Diversified | Level 2 Inputs
Investment securities
Fair value investments	3.5
Fair value measured on a recurring basis | Municipal obligations | Fair Value
Investment securities
Fair value investments	2.7	3.3
Fair value measured on a recurring basis | Municipal obligations | Level 2 Inputs
Investment securities
Fair value investments	2.7	3.3
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Fair Value
Investment securities
Fair value investments	3,222.9	2,071.7
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 2 Inputs
Investment securities
Fair value investments	3,207.8	2,014.7
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 3 Inputs
Investment securities
Fair value investments	15.1	57.0
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Fair Value
Investment securities
Fair value investments	589.2	1,046.5
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 1 Inputs
Investment securities
Fair value investments	65.7	82.6
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 2 Inputs
Investment securities
Fair value investments	523.5	963.9
Fair value measured on a recurring basis | Non-cumulative perpetual preference shares | Fair Value
Investment securities
Fair value investments	78.8	87.9
Fair value measured on a recurring basis | Non-cumulative perpetual preference shares | Level 2 Inputs
Investment securities
Fair value investments	15.0	16.5
Fair value measured on a recurring basis | Non-cumulative perpetual preference shares | Level 3 Inputs
Investment securities
Fair value investments	63.8	71.4
Fair value measured on a recurring basis | Common equity securities | Fair Value
Investment securities
Fair value investments	755.0	667.0
Fair value measured on a recurring basis | Common equity securities | Level 1 Inputs
Investment securities
Fair value investments	667.4	537.9
Fair value measured on a recurring basis | Common equity securities | Level 2 Inputs
Investment securities
Fair value investments	55.3	57.9
Fair value measured on a recurring basis | Common equity securities | Level 3 Inputs
Investment securities
Fair value investments	32.3	71.2
Fair value measured on a recurring basis | Common equity securities | Consumer | Fair Value
Investment securities
Fair value investments	188.8	123.3
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 1 Inputs
Investment securities
Fair value investments	188.5	122.5
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 2 Inputs
Investment securities
Fair value investments	0.3	0.8
Fair value measured on a recurring basis | Common equity securities | Financials | Fair Value
Investment securities
Fair value investments	219.2	221.6
Fair value measured on a recurring basis | Common equity securities | Financials | Level 1 Inputs
Investment securities
Fair value investments	185.8	152.6
Fair value measured on a recurring basis | Common equity securities | Financials | Level 2 Inputs
Investment securities
Fair value investments	1.5	1.2
Fair value measured on a recurring basis | Common equity securities | Financials | Level 3 Inputs
Investment securities
Fair value investments	31.9	67.8
Fair value measured on a recurring basis | Common equity securities | Basic materials | Fair Value
Investment securities
Fair value investments	121.0	98.8
Fair value measured on a recurring basis | Common equity securities | Basic materials | Level 1 Inputs
Investment securities
Fair value investments	119.9	97.1
Fair value measured on a recurring basis | Common equity securities | Basic materials | Level 2 Inputs
Investment securities
Fair value investments	1.1	1.7
Fair value measured on a recurring basis | Common equity securities | Utilities | Fair Value
Investment securities
Fair value investments	42.0	47.6
Fair value measured on a recurring basis | Common equity securities | Utilities | Level 1 Inputs
Investment securities
Fair value investments	41.8	44.6
Fair value measured on a recurring basis | Common equity securities | Utilities | Level 2 Inputs
Investment securities
Fair value investments	0.2
Fair value measured on a recurring basis | Common equity securities | Utilities | Level 3 Inputs
Investment securities
Fair value investments		3.0
Fair value measured on a recurring basis | Common equity securities | Energy | Fair Value
Investment securities
Fair value investments	72.6	60.2
Fair value measured on a recurring basis | Common equity securities | Energy | Level 1 Inputs
Investment securities
Fair value investments	72.6	60.2
Fair value measured on a recurring basis | Common equity securities | Technology | Fair Value
Investment securities
Fair value investments	25.8	31.8
Fair value measured on a recurring basis | Common equity securities | Technology | Level 1 Inputs
Investment securities
Fair value investments	25.8	30.4
Fair value measured on a recurring basis | Common equity securities | Technology | Level 2 Inputs
Investment securities
Fair value investments		1.4
Fair value measured on a recurring basis | Common equity securities | Other: | Fair Value
Investment securities
Fair value investments	85.6	83.7
Fair value measured on a recurring basis | Common equity securities | Other: | Level 1 Inputs
Investment securities
Fair value investments	33.0	30.5
Fair value measured on a recurring basis | Common equity securities | Other: | Level 2 Inputs
Investment securities
Fair value investments	52.2	52.8
Fair value measured on a recurring basis | Common equity securities | Other: | Level 3 Inputs
Investment securities
Fair value investments	0.4	0.4
Fair value measured on a recurring basis | Short-term investments | Fair Value
Investment securities
Fair value investments	846.0	975.3
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Investment securities
Fair value investments	846.0	878.7
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Investment securities
Fair value investments		96.6
Fair value measured on a recurring basis | Convertible fixed maturities | Fair Value
Investment securities
Fair value investments	143.8	143.0
Fair value measured on a recurring basis | Convertible fixed maturities | Level 2 Inputs
Investment securities
Fair value investments	143.8	143.0
Fair value measured on a recurring basis | Other long-term investments | Fair Value
Investment securities
Fair value investments	268.3	330.2
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Investment securities
Fair value investments	268.3	330.2
Carrying value of investment accounted for using the equity method	$ 33.0	$ 41.9