Fair Value Measurements (Details 3) (Recurring Fair Value Measurements, USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Available-for-sale \| Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	$ 65.6	$ 22.6	$ 134.5	$ 46.5
Transfers out of Level 3 into Level 2	48.7	15.0	72.3	122.5
Available-for-sale \| Non-U.S. governments
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3				14.5
Available-for-sale \| Corporate debt securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	53.1	22.6	100.6	26.0
Transfers out of Level 3 into Level 2	26.5	15.0	50.1	95.5
Available-for-sale \| Collateralized debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	10.3		31.7
Transfers out of Level 3 into Level 2	22.2		22.2	27.0
Available-for-sale \| Other debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	2.2		2.2	6.0
Separate account assets
Fair Value Hierarchy Levels Transfers
Transfers out of Level 1 into Level 2	0.4	3,222.8	243.8	3,222.8
Transfers out of Level 1 into Level 3	0.1		0.1	0.3
Transfers out of Level 2 into Level 1	0.2		4.8
Transfers out of Level 2 into Level 3	6.3	1.3	7.7	1.3
Transfers out of Level 3 into Level 2	0.2	0.1	0.2	1.8
Trading \| Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3		$ 9.5		$ 9.5

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Amount of transfers of assets measured on a recurring basis out of Level 1 into Level 2 of the fair value hierarchy.

No definition available.

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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 3.

No definition available.

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Amount of transfers of assets measured on a recurring basis out of Level 2 into Level 1 of the fair value hierarchy.

No definition available.

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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 3.

No definition available.

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Amount of transfers of assets measured on a recurring basis out of Level 3 of the fair value hierarchy into Level 2.

No definition available.

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