Fair Value Measurements - Transfers (Details) - Recurring Fair Value Measurements - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Available-for-sale \| Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3		$ 19.5
Transfers out of Level 3 into Level 2	$ 4.0	62.1
Available-for-sale \| Corporate debt securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3		19.5
Transfers out of Level 3 into Level 2	1.7	47.7
Available-for-sale \| Commercial mortgage-backed securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2	2.3
Available-for-sale \| Other debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2		14.4
Separate account assets
Fair Value Hierarchy Levels Transfers
Transfers out of Level 1 into Level 2	26.2	1.1
Transfers out of Level 2 into Level 1	4.5	0.6
Transfers out of Level 2 into Level 3	$ 0.8	0.1
Other investments
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2		$ 35.0

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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.

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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.

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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.

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