Derivative Financial Statement (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Cash deposits	$ 201.4	$ 405.3
Contractual Notional Amount	6,263.1	16,834.9
Unrecognized loss for all outstanding and terminated cash flow hedges included in accumulated other comprehensive income	236.1		$ 553.8
Cash Flow Hedging [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Contractual Notional Amount	4,375.0	4,775.0
Unrecognized loss for all outstanding and terminated cash flow hedges included in accumulated other comprehensive income	236.1	168.4
Total amount of amortization over the next twelve months for all cash flow hedges	15.4
Cash Flow Hedging [Member] | Expired hedge [Domain]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative Notional Amount Expired	5,875.0
Settlement amount received	473.9
Loss on Cash Flow Hedge Ineffectiveness	12.0
Non-Hedging Instrument [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Contractual Notional Amount	$ 653.1	$ 10,674.9