Fair Value Measurements - Transfers (Details) - Recurring Fair Value Measurements - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Available-for-sale \| Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	$ 5.3
Transfers out of Level 3 into Level 2	94.2	$ 4.0
Available-for-sale \| Corporate debt securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	4.6
Transfers out of Level 3 into Level 2		1.7
Available-for-sale \| Commercial mortgage-backed securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	0.7
Transfers out of Level 3 into Level 2	9.5	2.3
Available-for-sale \| Other debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2	84.7
Trading \| Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	1.9
Trading \| Equity securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	0.7
Separate account assets
Fair Value Hierarchy Levels Transfers
Transfers out of Level 1 into Level 2	0.7	26.2
Transfers out of Level 2 into Level 1		4.5
Transfers out of Level 2 into Level 3		$ 0.8
Transfers out of Level 3 into Level 2	$ 0.9

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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 into Level 1 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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