Investments - Balance Sheet Offsetting, Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Financial Liability Offsetting
Gross amount of recognized liabilities subject to netting agreements	$ 272.5	$ 577.2
Amount of assets that offset the gross amount of liabilities subject to netting agreements not offset in statement of financial position	(116.3)	(294.2)
Collateral pledged, financial liabilities	(143.5)	(253.6)
Net amount of liabilities subject to netting agreements	12.7	29.4
Derivative liabilities
Financial Liability Offsetting
Gross amount of recognized liabilities subject to netting agreements	272.5	567.5
Amount of assets that offset the gross amount of liabilities subject to netting agreements not offset in statement of financial position	(116.3)	(294.2)
Collateral pledged, financial liabilities	(143.5)	(243.9)
Net amount of liabilities subject to netting agreements	12.7	29.4
Gross amount of liabilities not subject to netting agreements	$ 415.6	394.3
Repurchase agreements
Financial Liability Offsetting
Gross amount of recognized liabilities subject to netting agreements		9.7
Collateral pledged, financial liabilities		$ (9.7)