Investment Securities (Other long-term investments) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Jun. 30, 2015	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Payments to Acquire Other Investments		$ 30.5	$ 26.8
Other Investments		294.6		$ 318.0
Partnership investments [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments		3.9		5.1
Hedge and private equity funds included in other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments		148.6		178.5
Surplus Note
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments		58.0		65.1
Limited Liability Companies and Private Equity Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments		65.3		45.3
Affordable Housing Development Fund [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments		15.2	$ 17.3	16.8	$ 19.1
Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments		3.6		7.2
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
Hedge Funds Equity R E I T [Member] | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments		20.0		20.4
Unfunded Commitments		$ 0.0		$ 0.0