Derivative Financial Statement (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Collateral posted	$ 115.6
Collateral received	63.3
Cash deposits	310.5	$ 405.3
Contractual Notional Amount	19,564.0	16,834.9
Unrecognized loss for all outstanding and terminated cash flow hedges included in accumulated other comprehensive income	151.4		$ 346.6
Cash Flow Hedging [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Contractual Notional Amount	4,775.0	4,775.0
Unrecognized loss for all outstanding and terminated cash flow hedges included in accumulated other comprehensive income	151.4	168.4
Total amount of amortization over the next twelve months for all cash flow hedges	8.6
Cash Flow Hedging [Member] | Expired hedge [Domain]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative Notional Amount Expired	450.0
Settlement amount received	74.6
Loss on Cash Flow Hedge Ineffectiveness	12.0
Non-Hedging Instrument [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Contractual Notional Amount	$ 13,554.0	$ 10,674.9