Derivative Financial Instruments - Fair Value of Derivatives (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Derivatives, fair value disclosures
Derivative instruments, assets	$ 321.2	$ 893.6
Derivative instruments, liabilities	645.1	961.8
Fair value of embedded derivative liabilities reported with contractholder funds	138.2	176.5
Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	51.8	91.2
Derivative instruments, liabilities	53.5	214.7
Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	269.4	802.4
Derivative instruments, liabilities	591.6	747.1
Interest rate contracts | Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	0.9	4.4
Derivative instruments, liabilities	27.4	71.3
Interest rate contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	196.9	739.3
Derivative instruments, liabilities	42.1	200.6
Foreign exchange contracts | Derivatives designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	50.9	86.8
Derivative instruments, liabilities	26.1	143.4
Foreign exchange contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	51.5	27.2
Derivative instruments, liabilities	32.3	56.2
Equity contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	15.4	28.2
Derivative instruments, liabilities	135.0	95.9
Credit contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, assets	5.6	7.7
Derivative instruments, liabilities	1.5	5.7
Other contracts | Derivatives not designated as hedging instruments
Derivatives, fair value disclosures
Derivative instruments, liabilities	$ 380.7	$ 388.7