Derivative Financial Instruments - Fair Value of Derivatives (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivatives, fair value disclosures
Derivative instruments, assets	$ 304.0	$ 893.6
Derivative instruments, liabilities	688.1	961.8
Fair value of embedded derivative liabilities reported with contractholder funds	160.3	176.5
Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	39.6	91.2
Derivative instruments, liabilities	59.2	214.7
Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	264.4	802.4
Derivative instruments, liabilities	628.9	747.1
Interest rate contracts | Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets		4.4
Derivative instruments, liabilities	22.9	71.3
Interest rate contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	175.2	739.3
Derivative instruments, liabilities	33.6	200.6
Foreign exchange contracts | Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	39.6	86.8
Derivative instruments, liabilities	36.3	143.4
Foreign exchange contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	66.2	27.2
Derivative instruments, liabilities	26.3	56.2
Equity contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	18.2	28.2
Derivative instruments, liabilities	154.1	95.9
Credit contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	4.8	7.7
Derivative instruments, liabilities	1.5	5.7
Other contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, liabilities	$ 413.4	$ 388.7