Accounting for Derivative Instruments and Hedging Activities (Schedule of Outstanding Contracts) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Derivative [Line Items]
Notional amount of interest rate swaps	$ 450.0
Notional amount of interest rate swaps	$ 450.0
Gas Derivatives | Not Designated as Hedging Instrument
Derivative [Line Items]
Nonmonetary notional amount of derivatives	516,909,006
Electric Generation Fuel | Not Designated as Hedging Instrument
Derivative [Line Items]
Nonmonetary notional amount of derivatives	129,693,200
Purchased Electricity | Not Designated as Hedging Instrument
Derivative [Line Items]
Nonmonetary notional amount of derivatives	10,722,415