Investments (Details 16) (Derivative liabilities, USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivative liabilities
Financial Liability Offsetting
Gross amount of recognized liabilities subject to netting agreements	$ 904.5	$ 786.0
Amount of assets that offset the gross amount of liabilities subject to netting agreements not offset in statement of financial position	(542.5)	(479.5)
Collateral pledged, financial liabilities	(269.1)	(220.6)
Net amount of liabilities subject to netting agreements	92.9	85.9
Gross amount of liabilities not subject to netting agreements	$ 469.0	$ 421.3