Fair Value Measurements (Details 3) (Recurring Fair Value Measurements, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Available-for-sale | Fixed maturities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	$ 19.5	$ 6.6
Transfers out of Level 3 into Level 2	62.1	37.2
Available-for-sale | Corporate debt securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3	19.5	0.5
Transfers out of Level 3 into Level 2	47.7
Available-for-sale | Commercial mortgage-backed securities
Fair Value Hierarchy Levels Transfers
Transfers out of Level 2 into Level 3		6.1
Available-for-sale | Collateralized debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2		7.5
Available-for-sale | Other debt obligations
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2	14.4	29.7
Separate account assets
Fair Value Hierarchy Levels Transfers
Transfers out of Level 1 into Level 2	1.1	3.7
Transfers out of Level 2 into Level 1	0.6	53.2
Transfers out of Level 2 into Level 3	0.1	2.0
Transfers out of Level 3 into Level 2		5.8
Other investments
Fair Value Hierarchy Levels Transfers
Transfers out of Level 3 into Level 2	$ 35.0