Fair Value Measurements (Details 3) (Recurring Fair Value Measurements, USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Available-for-sale \| Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	$ 68.9	$ 23.9
Transfers out of Level 3 into Level 2	23.6	107.5
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	47.5	3.4
Transfers out of Level 3 into Level 2	23.6	80.5
Available-for-sale \| Collateralized debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	21.4
Transfers out of Level 3 into Level 2		27.0
Available-for-sale \| Other debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3		6.0
Separate account assets
Fair Value Hierarchy Levels Transfers
Transfers out of Level 1 into Level 2	243.4
Transfers out of Level 1 into Level 3		0.3
Transfers out of Level 2 into Level 1	4.6
Transfers out of Level 2 into Level 3	1.4
Transfers out of Level 3 into Level 2		$ 1.7

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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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