Investments - Balance Sheet Offsetting, Assets (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Financial Asset Offsetting
Gross amount of recognized assets subject to netting agreements	$ 745.1	$ 713.3
Amount of liabilities that offset the gross amount of assets subject to netting agreements not offset in statement of financial position	(409.7)	(479.5)
Collateral received, financial assets	(313.3)	(220.5)
Net amount of assets subject to netting agreements	22.1	13.3
Derivative assets
Financial Asset Offsetting
Gross amount of recognized assets subject to netting agreements	665.4	661.8
Amount of liabilities that offset the gross amount of assets subject to netting agreements not offset in statement of financial position	(409.7)	(479.5)
Collateral received, financial assets	(233.6)	(169.0)
Net amount of assets subject to netting agreements	22.1	13.3
Gross amount of assets not subject to netting agreements	1.2	0.0
Reverse repurchase agreements
Financial Asset Offsetting
Gross amount of recognized assets subject to netting agreements	79.7	51.5
Collateral received, financial assets	$ (79.7)	$ (51.5)