Investment Securities (Other long-term investments) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Payments to Acquire Other Investments		$ 33.3	$ 17.7
Other Investments	$ 390.1	390.1		$ 407.0
Partnership investments [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	3.9	3.9		5.2
Hedge and private equity funds included in other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	210.8	210.8		242.9
Surplus Note
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	66.3	66.3		65.1
Limited Liability Companies and Private Equity Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	89.9	89.9		69.7
Affordable Housing Development Fund [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	15.8	15.8	$ 18.0	16.8	$ 19.1
Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	3.4	3.4		7.3
PassportCard [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Payments to Acquire Other Investments	21.0
Hedge Funds, Equity Long (Short) [Member] | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unfunded Commitments	0.0	0.0		0.0
Hedge Funds Equity R E I T [Member] | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	20.5	20.5		20.3
Unfunded Commitments	$ 0.0	$ 0.0		$ 0.0