Derivative Financial Instruments - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Offsetting Assets and liabilities [Line Items]
Asset Fair Value	$ 438.5	$ 562.0
Liability Fair Value	144.4	216.9
Counterparty netting, Assets	(142.0)	(216.2)
Counterparty netting, Liabilities	(142.0)	(216.2)
Cash collateral netting, Assets	(253.7)	(291.5)
Cash collateral netting, Liabilities	(0.1)	0.0
Securities collateral netting, Assets	(4.3)	(6.6)
Securities collateral netting, Liabilities	(0.9)	0.0
Net receivables	38.5	47.7
Net payables	1.4	0.7
Credit contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	407.5	384.0
Asset Fair Value	5.2	6.5
Liability Fair Value	0.2	0.0
Foreign exchange contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	181.3	181.3
Asset Fair Value	25.1	18.3
Liability Fair Value	10.6	7.7
Interest rate contracts
Offsetting Assets and liabilities [Line Items]
Notional Amount	22,007.7	28,146.2
Asset Fair Value	408.2	537.2
Liability Fair Value	$ 133.6	$ 209.2