Investments - Balance Sheet Offsetting, Assets (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Financial Asset Offsetting
Gross amount of recognized assets subject to netting agreements	$ 246.1	$ 304.6
Amount of liabilities that offset the gross amount of assets subject to netting agreements not offset in statement of financial position	(100.7)	(116.3)
Collateral received, financial assets	(138.3)	(167.1)
Net amount of assets subject to netting agreements	7.1	21.2
Derivative assets
Financial Asset Offsetting
Gross amount of recognized assets subject to netting agreements	217.6	287.0
Amount of liabilities that offset the gross amount of assets subject to netting agreements not offset in statement of financial position	(100.7)	(116.3)
Collateral received, financial assets	(109.8)	(149.5)
Net amount of assets subject to netting agreements	7.1	21.2
Gross amount of assets not subject to netting agreements	19.0	17.0
Reverse repurchase agreements
Financial Asset Offsetting
Gross amount of recognized assets subject to netting agreements	28.5	17.6
Collateral received, financial assets	$ (28.5)	$ (17.6)