Investments - Balance Sheet Offsetting, Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Financial Asset Offsetting
Gross amount of recognized assets subject to netting agreements	$ 304.6	$ 928.3
Amount of liabilities that offset the gross amount of assets subject to netting agreements not offset in statement of financial position	(116.3)	(294.2)
Collateral received, financial assets	(167.1)	(623.1)
Net amount of assets subject to netting agreements	21.2	11.0
Derivative assets
Financial Asset Offsetting
Gross amount of recognized assets subject to netting agreements	287.0	887.2
Amount of liabilities that offset the gross amount of assets subject to netting agreements not offset in statement of financial position	(116.3)	(294.2)
Collateral received, financial assets	(149.5)	(582.0)
Net amount of assets subject to netting agreements	21.2	11.0
Gross amount of assets not subject to netting agreements	17.0	6.4
Reverse repurchase agreements
Financial Asset Offsetting
Gross amount of recognized assets subject to netting agreements	17.6	41.1
Collateral received, financial assets	$ (17.6)	$ (41.1)