Derivative Financial Instruments - Fair Value of Derivatives (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Derivatives, fair value disclosures
Derivative instruments, assets	$ 294.7	$ 194.0
Derivative instruments, liabilities	530.5	291.7
Fair value of embedded derivative liabilities reported with contractholder funds	214.2	45.2
Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	30.0	37.6
Derivative instruments, liabilities	36.5	29.6
Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	264.7	156.4
Derivative instruments, liabilities	494.0	262.1
Interest rate contracts | Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, liabilities	21.3	16.1
Interest rate contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	204.2	108.0
Derivative instruments, liabilities	16.7	22.6
Foreign exchange contracts | Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	30.0	37.6
Derivative instruments, liabilities	15.2	13.5
Foreign exchange contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	29.5	38.4
Derivative instruments, liabilities	100.2	72.9
Equity contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	30.5	7.7
Derivative instruments, liabilities	63.1	27.6
Credit contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	0.5	2.3
Derivative instruments, liabilities	1.2	4.4
Other contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, liabilities	$ 312.8	$ 134.6