Derivative Financial Instruments - Net Realized Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	$ 8.1	$ (3.3)	$ 7.0	$ (108.1)
Designated as Hedging Instrument | Cash flow hedges | Other Net Realized Capital Gains (Losses) | Interest rate contracts
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	0.1	0.2	0.3	0.5
Designated as Hedging Instrument | Cash flow hedges | Other Net Realized Capital Gains (Losses) | Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	2.7	0.2	10.5	0.5
Not Designated as Hedging Instrument | Other Net Realized Capital Gains (Losses) | Credit contracts
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	1.4	0.9	4.2	2.0
Not Designated as Hedging Instrument | Other Net Realized Capital Gains (Losses) | Interest rate contracts
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	0.9	(6.7)	(4.0)	70.7
Not Designated as Hedging Instrument | Other Net Realized Capital Gains (Losses) | Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	(1.0)	(0.7)	(1.4)	(4.4)
Not Designated as Hedging Instrument | Other Net Realized Capital Gains (Losses) | Equity contract
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	0.3	0.7	1.1	0.4
Not Designated as Hedging Instrument | Other Net Realized Capital Gains (Losses) | Fixed maturities
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	(1.0)	(1.6)	(3.5)	(1.3)
Not Designated as Hedging Instrument | Other Net Realized Capital Gains (Losses) | Within products
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	7.1	4.6	18.0	(99.4)
Not Designated as Hedging Instrument | Other Net Realized Capital Gains (Losses) | Managed custody guarantees
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	0.1	(0.1)	0.2	(4.0)
Not Designated as Hedging Instrument | Interest Credited and Other Benefits to Contract Owners | Reinsurance agreements
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	$ (2.5)	$ (0.8)	$ (18.4)	$ (73.1)