Derivative Financial Instruments - Fair Value of Derivatives (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Derivatives, fair value disclosures
Derivative instruments, assets	$ 291.5	$ 194.0
Derivative instruments, liabilities	381.9	291.7
Fair value of embedded derivative liabilities reported with contractholder funds	118.7	45.2
Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	29.2	37.6
Derivative instruments, liabilities	32.2	29.6
Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	262.3	156.4
Derivative instruments, liabilities	349.7	262.1
Interest rate contracts | Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, liabilities	22.0	16.1
Interest rate contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	198.9	108.0
Derivative instruments, liabilities	19.7	22.6
Foreign exchange contracts | Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	29.2	37.6
Derivative instruments, liabilities	10.2	13.5
Foreign exchange contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	40.3	38.4
Derivative instruments, liabilities	63.0	72.9
Equity contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	22.2	7.7
Derivative instruments, liabilities	51.9	27.6
Credit contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	0.9	2.3
Derivative instruments, liabilities	1.1	4.4
Other contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, liabilities	$ 214.0	$ 134.6