Investments - Balance Sheet Offsetting, Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Financial Liability Offsetting
Gross amount of recognized liabilities subject to netting agreements	$ 275.2	$ 577.2
Amount of assets that offset the gross amount of liabilities subject to netting agreements not offset in statement of financial position	(121.2)	(294.2)
Collateral pledged, financial liabilities	(129.7)	(253.6)
Net amount of liabilities subject to netting agreements	24.3	29.4
Derivative liabilities
Financial Liability Offsetting
Gross amount of recognized liabilities subject to netting agreements	260.1	567.5
Amount of assets that offset the gross amount of liabilities subject to netting agreements not offset in statement of financial position	(121.2)	(294.2)
Collateral pledged, financial liabilities	(114.6)	(243.9)
Net amount of liabilities subject to netting agreements	24.3	29.4
Gross amount of liabilities not subject to netting agreements	385.0	394.3
Repurchase agreements
Financial Liability Offsetting
Gross amount of recognized liabilities subject to netting agreements	15.1	9.7
Collateral pledged, financial liabilities	$ (15.1)	$ (9.7)