Investments - Balance Sheet Offsetting, Assets (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Financial Asset Offsetting
Gross amount of recognized assets subject to netting agreements	$ 355.0	$ 239.3
Amount of liabilities that offset the gross amount of assets subject to netting agreements not offset in statement of financial position	(80.9)	(70.5)
Collateral received, financial assets	(262.9)	(161.1)
Net amount of assets subject to netting agreements	11.2	7.7
Derivative assets
Financial Asset Offsetting
Gross amount of recognized assets subject to netting agreements	280.4	186.3
Amount of liabilities that offset the gross amount of assets subject to netting agreements not offset in statement of financial position	(80.9)	(70.5)
Collateral received, financial assets	(188.3)	(108.1)
Net amount of assets subject to netting agreements	11.2	7.7
Gross amount of assets not subject to netting agreements	11.1	7.7
Reverse repurchase agreements
Financial Asset Offsetting
Gross amount of recognized assets subject to netting agreements	74.6	53.0
Collateral received, financial assets	$ (74.6)	$ (53.0)