Derivative Financial Instruments - Fair Value of Derivatives (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Derivatives, fair value disclosures
Derivative instruments, assets	$ 463.5	$ 294.7
Derivative instruments, liabilities	654.0	530.5
Fair value of embedded derivative liabilities reported with contractholder funds	467.8	214.2
Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	21.1	30.0
Derivative instruments, liabilities	72.5	36.5
Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	442.4	264.7
Derivative instruments, liabilities	581.5	494.0
Interest rate contracts | Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, liabilities	27.8	21.3
Interest rate contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	339.3	204.2
Derivative instruments, liabilities	33.0	16.7
Foreign exchange contracts | Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	21.1	30.0
Derivative instruments, liabilities	44.7	15.2
Foreign exchange contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	66.5	29.5
Derivative instruments, liabilities	29.2	100.2
Equity contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	33.2	30.5
Derivative instruments, liabilities	49.0	63.1
Credit contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	3.4	0.5
Derivative instruments, liabilities	2.5	1.2
Other contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, liabilities	$ 467.8	$ 312.8