Investments - Balance Sheet Offsetting, Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Financial Liability Offsetting
Gross amount of recognized liabilities subject to netting agreements	$ 304.7	$ 577.2
Amount of assets that offset the gross amount of liabilities subject to netting agreements not offset in statement of financial position	(147.1)	(294.2)
Collateral pledged, financial liabilities	(115.5)	(243.9)
Net amount of liabilities subject to netting agreements	42.1	39.1
Derivative liabilities
Financial Liability Offsetting
Gross amount of recognized liabilities subject to netting agreements	287.3	567.5
Amount of assets that offset the gross amount of liabilities subject to netting agreements not offset in statement of financial position	(147.1)	(294.2)
Collateral pledged, financial liabilities	(115.5)	(243.9)
Net amount of liabilities subject to netting agreements	24.7	29.4
Gross amount of liabilities not subject to netting agreements	347.5	394.3
Repurchase agreements
Financial Liability Offsetting
Gross amount of recognized liabilities subject to netting agreements	17.4	9.7
Net amount of liabilities subject to netting agreements	$ 17.4	$ 9.7