Derivative Financial Instruments - Fair Value of Derivatives (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Derivatives, fair value disclosures
Derivative instruments, assets	$ 475.6	$ 294.7
Derivative instruments, liabilities	712.7	530.5
Fair value of embedded derivative liabilities reported with contractholder funds	552.4	214.2
Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	42.1	30.0
Derivative instruments, liabilities	38.8	36.5
Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	433.5	264.7
Derivative instruments, liabilities	673.9	494.0
Interest rate contracts | Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, liabilities	29.9	21.3
Interest rate contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	377.5	204.2
Derivative instruments, liabilities	37.3	16.7
Foreign exchange contracts | Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	42.1	30.0
Derivative instruments, liabilities	8.9	15.2
Foreign exchange contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	18.2	29.5
Derivative instruments, liabilities	60.4	100.2
Equity contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	34.9	30.5
Derivative instruments, liabilities	21.3	63.1
Credit contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	2.9	0.5
Derivative instruments, liabilities	2.5	1.2
Other contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, liabilities	$ 552.4	$ 312.8