Derivative Financial Instruments - Fair Value of Derivatives (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Derivatives, fair value disclosures
Derivative instruments, assets	$ 531.4	$ 294.7
Derivative instruments, liabilities	768.8	530.5
Fair value of embedded derivative liabilities reported with contractholder funds	576.0	214.2
Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	61.3	30.0
Derivative instruments, liabilities	32.0	36.5
Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	470.1	264.7
Derivative instruments, liabilities	736.8	494.0
Interest rate contracts | Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, liabilities	31.2	21.3
Interest rate contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	408.3	204.2
Derivative instruments, liabilities	40.9	16.7
Foreign exchange contracts | Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	61.3	30.0
Derivative instruments, liabilities	0.8	15.2
Foreign exchange contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	20.0	29.5
Derivative instruments, liabilities	102.9	100.2
Equity contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	39.3	30.5
Derivative instruments, liabilities	14.7	63.1
Credit contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	2.5	0.5
Derivative instruments, liabilities	2.3	1.2
Other contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, liabilities	$ 576.0	$ 312.8