Derivative Financial Instruments - Fair Value of Derivatives (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Derivatives, fair value disclosures
Derivative instruments, assets	$ 236.6	$ 304.0
Derivative instruments, liabilities	618.3	688.1
Fair value of embedded derivative liabilities reported with contractholder funds	96.2	160.3
Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	32.4	39.6
Derivative instruments, liabilities	61.6	59.2
Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	204.2	264.4
Derivative instruments, liabilities	556.7	628.9
Interest rate contracts | Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, liabilities	18.3	22.9
Interest rate contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	125.8	175.2
Derivative instruments, liabilities	31.2	33.6
Foreign exchange contracts | Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	32.4	39.6
Derivative instruments, liabilities	43.3	36.3
Foreign exchange contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	58.9	66.2
Derivative instruments, liabilities	22.8	26.3
Equity contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	15.4	18.2
Derivative instruments, liabilities	144.1	154.1
Credit contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	4.1	4.8
Derivative instruments, liabilities	1.4	1.5
Other contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, liabilities	$ 357.2	$ 413.4