Investment Securities (Details 6) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Investment Securities
Fair value, investment-related liabilities	$ 86.4	$ 77.8
Fixed maturity investments | Level 3 Inputs | Fair value measured on a recurring basis
Investment securities
Fair value investments	123.0	128.4
Fixed maturity investments | Fair value measured on a recurring basis | Fair Value
Investment securities
Fair value investments	5,558.9	5,786.1
Fixed maturity investments | Fair value measured on a recurring basis | Level 1 Inputs
Investment securities
Fair value investments	376.9	477.8
Fixed maturity investments | Fair value measured on a recurring basis | Level 2 Inputs
Investment securities
Fair value investments	5,059.0	5,179.9
Short-term investments | Fair value measured on a recurring basis | Fair Value
Investment securities
Fair value investments	813.5	975.3
Short-term investments | Fair value measured on a recurring basis | Level 1 Inputs
Investment securities
Fair value investments	723.4	878.7
Short-term investments | Fair value measured on a recurring basis | Level 2 Inputs
Investment securities
Fair value investments	90.1	96.6
U.S. Government and agency obligations | Fair value measured on a recurring basis | Fair Value
Investment securities
Fair value investments	296.2	398.7
U.S. Government and agency obligations | Fair value measured on a recurring basis | Level 1 Inputs
Investment securities
Fair value investments	286.6	388.9
U.S. Government and agency obligations | Fair value measured on a recurring basis | Level 2 Inputs
Investment securities
Fair value investments	9.6	9.8
Common Equity Securities | Level 3 Inputs | Fair value measured on a recurring basis
Investment securities
Fair value investments	74.5	71.2
Common Equity Securities | Level 3 Inputs | Fair value measured on a recurring basis | Consumer
Investment securities
Fair value investments	0.5
Common Equity Securities | Level 3 Inputs | Fair value measured on a recurring basis | Financials
Investment securities
Fair value investments	69.6	67.8
Common Equity Securities | Level 3 Inputs | Fair value measured on a recurring basis | Utilities
Investment securities
Fair value investments	4.0	3.0
Common Equity Securities | Level 3 Inputs | Fair value measured on a recurring basis | Other:
Investment securities
Fair value investments	0.4	0.4
Common Equity Securities | Fair value measured on a recurring basis | Consumer | Fair Value
Investment securities
Fair value investments	150.8	123.3
Common Equity Securities | Fair value measured on a recurring basis | Consumer | Level 1 Inputs
Investment securities
Fair value investments	149.1	122.5
Common Equity Securities | Fair value measured on a recurring basis | Consumer | Level 2 Inputs
Investment securities
Fair value investments	1.2	0.8
Common Equity Securities | Fair value measured on a recurring basis | Financials | Fair Value
Investment securities
Fair value investments	239.6	221.6
Common Equity Securities | Fair value measured on a recurring basis | Financials | Level 1 Inputs
Investment securities
Fair value investments	169.4	152.6
Common Equity Securities | Fair value measured on a recurring basis | Financials | Level 2 Inputs
Investment securities
Fair value investments	0.6	1.2
Common Equity Securities | Fair value measured on a recurring basis | Energy | Fair Value
Investment securities
Fair value investments	64.6	60.2
Common Equity Securities | Fair value measured on a recurring basis | Energy | Level 1 Inputs
Investment securities
Fair value investments	64.6	60.2
Common Equity Securities | Fair value measured on a recurring basis | Basic materials | Fair Value
Investment securities
Fair value investments	91.7	98.8
Common Equity Securities | Fair value measured on a recurring basis | Basic materials | Level 1 Inputs
Investment securities
Fair value investments	90.5	97.1
Common Equity Securities | Fair value measured on a recurring basis | Basic materials | Level 2 Inputs
Investment securities
Fair value investments	1.2	1.7
Common Equity Securities | Fair value measured on a recurring basis | Utilities | Fair Value
Investment securities
Fair value investments	46.3	47.6
Common Equity Securities | Fair value measured on a recurring basis | Utilities | Level 1 Inputs
Investment securities
Fair value investments	42.3	44.6
Common Equity Securities | Fair value measured on a recurring basis | Technology | Fair Value
Investment securities
Fair value investments	34.4	31.8
Common Equity Securities | Fair value measured on a recurring basis | Technology | Level 1 Inputs
Investment securities
Fair value investments	33.5	30.4
Common Equity Securities | Fair value measured on a recurring basis | Technology | Level 2 Inputs
Investment securities
Fair value investments	0.9	1.4
Common Equity Securities | Fair value measured on a recurring basis | Other: | Fair Value
Investment securities
Fair value investments	76.4	83.7
Common Equity Securities | Fair value measured on a recurring basis | Other: | Level 1 Inputs
Investment securities
Fair value investments	18.1	30.5
Common Equity Securities | Fair value measured on a recurring basis | Other: | Level 2 Inputs
Investment securities
Fair value investments	57.9	52.8
Common Equity Securities | Fair value measured on a recurring basis | Fair Value
Investment securities
Fair value investments	703.8	667.0
Common Equity Securities | Fair value measured on a recurring basis | Level 1 Inputs
Investment securities
Fair value investments	567.5	537.9
Common Equity Securities | Fair value measured on a recurring basis | Level 2 Inputs
Investment securities
Fair value investments	61.8	57.9
Debt securities issued by corporations | Fair value measured on a recurring basis | Consumer | Fair Value
Investment securities
Fair value investments	708.6	769.0
Debt securities issued by corporations | Fair value measured on a recurring basis | Consumer | Level 2 Inputs
Investment securities
Fair value investments	708.6	769.0
Debt securities issued by corporations | Fair value measured on a recurring basis | Industrial | Fair Value
Investment securities
Fair value investments	452.7	511.3
Debt securities issued by corporations | Fair value measured on a recurring basis | Industrial | Level 2 Inputs
Investment securities
Fair value investments	452.7	511.3
Debt securities issued by corporations | Fair value measured on a recurring basis | Financials | Fair Value
Investment securities
Fair value investments	239.7	265.8
Debt securities issued by corporations | Fair value measured on a recurring basis | Financials | Level 1 Inputs
Investment securities
Fair value investments	6.4	6.3
Debt securities issued by corporations | Fair value measured on a recurring basis | Financials | Level 2 Inputs
Investment securities
Fair value investments	233.3	259.5
Debt securities issued by corporations | Fair value measured on a recurring basis | Communications | Fair Value
Investment securities
Fair value investments	220.5	226.3
Debt securities issued by corporations | Fair value measured on a recurring basis | Communications | Level 2 Inputs
Investment securities
Fair value investments	220.5	226.3
Debt securities issued by corporations | Fair value measured on a recurring basis | Energy | Fair Value
Investment securities
Fair value investments	87.5	151.5
Debt securities issued by corporations | Fair value measured on a recurring basis | Energy | Level 2 Inputs
Investment securities
Fair value investments	87.5	151.5
Debt securities issued by corporations | Fair value measured on a recurring basis | Basic materials | Fair Value
Investment securities
Fair value investments	159.4	125.3
Debt securities issued by corporations | Fair value measured on a recurring basis | Basic materials | Level 2 Inputs
Investment securities
Fair value investments	159.4	125.3
Debt securities issued by corporations | Fair value measured on a recurring basis | Utilities | Fair Value
Investment securities
Fair value investments	141.9	106.7
Debt securities issued by corporations | Fair value measured on a recurring basis | Utilities | Level 2 Inputs
Investment securities
Fair value investments	141.9	106.7
Debt securities issued by corporations | Fair value measured on a recurring basis | Technology | Fair Value
Investment securities
Fair value investments	22.5	22.1
Debt securities issued by corporations | Fair value measured on a recurring basis | Technology | Level 2 Inputs
Investment securities
Fair value investments	22.5	22.1
Debt securities issued by corporations | Fair value measured on a recurring basis | Fair Value
Investment securities
Fair value investments	2,032.8	2,178.0
Debt securities issued by corporations | Fair value measured on a recurring basis | Level 1 Inputs
Investment securities
Fair value investments	6.4	6.3
Debt securities issued by corporations | Fair value measured on a recurring basis | Level 2 Inputs
Investment securities
Fair value investments	2,026.4	2,171.7
Convertible fixed maturity investments | Fair value measured on a recurring basis | Fair Value
Investment securities
Fair value investments	119.7	143.0
Convertible fixed maturity investments | Fair value measured on a recurring basis | Level 2 Inputs
Investment securities
Fair value investments	119.7	143.0
Municipal obligations | Fair value measured on a recurring basis | Fair Value
Investment securities
Fair value investments	3.3	3.3
Municipal obligations | Fair value measured on a recurring basis | Level 2 Inputs
Investment securities
Fair value investments	3.3	3.3
Mortgage-backed and asset-backed securities | Level 3 Inputs | Fair value measured on a recurring basis
Investment securities
Fair value investments	52.8	57.0
Mortgage-backed and asset-backed securities | Fair value measured on a recurring basis | Fair Value
Investment securities
Fair value investments	2,308.0	2,071.7
Mortgage-backed and asset-backed securities | Fair value measured on a recurring basis | Level 2 Inputs
Investment securities
Fair value investments	2,255.2	2,014.7
Other long-term investments. | Level 3 Inputs | Fair value measured on a recurring basis
Investment securities
Fair value investments	298.0	330.2
Carrying value of investment accounted for using the equity method	35.3	41.9
Other long-term investments. | Fair value measured on a recurring basis | Fair Value
Investment securities
Fair value investments	298.0	330.2
Non-cumulative perpetual preference shares | Level 3 Inputs | Fair value measured on a recurring basis
Investment securities
Fair value investments	70.2	71.4
Non-cumulative perpetual preference shares | Fair value measured on a recurring basis | Fair Value
Investment securities
Fair value investments	87.1	87.9
Non-cumulative perpetual preference shares | Fair value measured on a recurring basis | Level 2 Inputs
Investment securities
Fair value investments	16.9	16.5
Level 3 Inputs | Fair value measured on a recurring basis
Investment securities
Fair value investments	495.5	529.8
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations. | Fair Value
Investment securities
Fair value investments	831.5	1,046.5
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations. | Level 1 Inputs
Investment securities
Fair value investments	83.9	82.6
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations. | Level 2 Inputs
Investment securities
Fair value investments	747.6	963.9
Fair value measured on a recurring basis | Fair Value
Investment securities
Fair value investments	7,493.9	7,901.6
Fair value measured on a recurring basis | Level 1 Inputs
Investment securities
Fair value investments	1,667.8	1,894.4
Fair value measured on a recurring basis | Level 2 Inputs
Investment securities
Fair value investments	5,330.6	5,477.4
Fixed maturity investments | Level 3 Inputs
Investment securities
Fair value investments		128.4
Debt securities issued by corporations
Investment securities
Fair value investments	2,032.8	2,178.0
Mortgage-backed and asset-backed securities | Fair Value
Investment securities
Fair value investments	2,308.0	2,071.7
Mortgage-backed and asset-backed securities | Level 3 Inputs
Investment securities
Fair value investments	52.8	57.0
Mortgage-backed and asset-backed securities | Level 2 Inputs
Investment securities
Fair value investments	2,255.2	2,014.7
Common shareholders' equity | Level 3 Inputs
Investment securities
Fair value investments	74.5	71.2
Common Equity Securities | Level 3 Inputs
Investment securities
Fair value investments	74.5	71.2
Other long-term investments. | Level 3 Inputs
Investment securities
Fair value investments	298.0	330.2
Carrying value of investment accounted for using the equity method	35.3	41.9
Fair Value
Investment securities
Fair value investments	7,493.9	7,901.6
Level 1 Inputs
Investment securities
Fair value investments	1,667.8	1,894.4
Level 2 Inputs
Investment securities
Fair value investments	$ 5,330.6	$ 5,477.4