Derivative Financial Instruments (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Collateral posted	$ 92.4	$ 182.7
Collateral received	591.1	32.2
Cash deposits	405.3	79.9
Derivative, notional amount	16,834.9	23,376.1
Unrecognized losses for all terminated cash flow hedges included in accumulated other comprehensive loss	168.4	81.1
Cash Flow Hedging [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, notional amount	4,775.0	4,650.0
Unrecognized losses for all terminated cash flow hedges included in accumulated other comprehensive loss	168.4	81.1
Total amount of amortization over the next twelve months for all cash flow hedges	9.9
Cash Flow Hedging [Member] | Expired hedge [Domain]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative Notional Amount Expired	5,900.0
Settlement amount	745.7
Loss on Cash Flow Hedge Ineffectiveness	7.7
Derivative Instruments, Loss Recognized in Other Comprehensive Income (Loss), Effective Portion	738.0
Cash Flow Hedging [Member] | New hedge [Domain]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative Instruments, Gain Recognized in Other Comprehensive Income (Loss), Effective Portion	522.8
Non-hedging Instrument [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, notional amount	10,674.9	$ 17,341.1
Non-hedging Instrument [Member] | Expired hedge [Domain]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Settlement amount	$ 164.4