Investments - Balance Sheet Offsetting, Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Financial Liability Offsetting
Gross amount of recognized liabilities subject to netting agreements	$ 271.1	$ 272.5
Amount of assets that offset the gross amount of liabilities subject to netting agreements not offset in statement of financial position	(94.2)	(116.3)
Collateral pledged, financial liabilities	(153.2)	(143.5)
Net amount of liabilities subject to netting agreements	23.7	12.7
Derivative liabilities
Financial Liability Offsetting
Gross amount of recognized liabilities subject to netting agreements	263.1	272.5
Amount of assets that offset the gross amount of liabilities subject to netting agreements not offset in statement of financial position	(94.2)	(116.3)
Collateral pledged, financial liabilities	(145.2)	(143.5)
Net amount of liabilities subject to netting agreements	23.7	12.7
Gross amount of liabilities not subject to netting agreements	275.6	$ 415.6
Repurchase agreements
Financial Liability Offsetting
Gross amount of recognized liabilities subject to netting agreements	8.0
Collateral pledged, financial liabilities	$ (8.0)