Fair Value Accounting, Effect of Derivatives on Consolidated Statements of Income (Details) (USD $)			3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended		9 Months Ended				3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2010 Interest Rate Swap [Member] Interest Expense [Member] Derivatives Designated as Hedging Instruments [Member]	Sep. 30, 2010 Interest Rate Swap [Member] Interest Expense [Member] Derivatives Designated as Hedging Instruments [Member]	Sep. 30, 2010 Interest Rate Swap [Member] Derivatives Designated as Hedging Instruments [Member] OCI [Member]	Sep. 30, 2010 Interest Rate Swap [Member] Derivatives Designated as Hedging Instruments [Member] OCI [Member]	Sep. 30, 2011 Treasury Rate Locks [Member] Interest Expense [Member] Derivatives Designated as Hedging Instruments [Member]	Sep. 30, 2010 Treasury Rate Locks [Member] Interest Expense [Member] Derivatives Designated as Hedging Instruments [Member]	Sep. 30, 2011 Treasury Rate Locks [Member] Interest Expense [Member] Derivatives Designated as Hedging Instruments [Member]	Sep. 30, 2010 Treasury Rate Locks [Member] Interest Expense [Member] Derivatives Designated as Hedging Instruments [Member]	Sep. 30, 2011 Treasury Rate Locks [Member] Derivatives Designated as Hedging Instruments [Member] OCI [Member]	Sep. 30, 2011 Treasury Rate Locks [Member] Derivatives Designated as Hedging Instruments [Member] OCI [Member]	Sep. 30, 2011 Derivatives Not Designated As Hedging Instruments [Member]	Sep. 30, 2010 Derivatives Not Designated As Hedging Instruments [Member]	Sep. 30, 2011 Derivatives Not Designated As Hedging Instruments [Member]	Sep. 30, 2010 Derivatives Not Designated As Hedging Instruments [Member]	Sep. 30, 2011 Derivatives Not Designated As Hedging Instruments [Member] Fuel Cost Hedges [Member]	Dec. 31, 2010 Derivatives Not Designated As Hedging Instruments [Member] Fuel Cost Hedges [Member]	Sep. 30, 2011 Derivatives Not Designated As Hedging Instruments [Member] Fuel Cost Hedges [Member] Fuel Used For Electric Generation [Member]	Sep. 30, 2010 Derivatives Not Designated As Hedging Instruments [Member] Fuel Cost Hedges [Member] Fuel Used For Electric Generation [Member]	Sep. 30, 2011 Derivatives Not Designated As Hedging Instruments [Member] Fuel Cost Hedges [Member] Fuel Used For Electric Generation [Member]	Sep. 30, 2010 Derivatives Not Designated As Hedging Instruments [Member] Fuel Cost Hedges [Member] Fuel Used For Electric Generation [Member]	Sep. 30, 2010 Derivatives Not Designated As Hedging Instruments [Member] Economic Hedges [Member] Other Operations Revenue [Member]	Sep. 30, 2010 Derivatives Not Designated As Hedging Instruments [Member] Economic Hedges [Member] Other Operations Revenue [Member]
Derivative, gain (loss) recognized in income [Abstract]
Amount of gain (loss) recognized in income on derivatives													$ (5,678,000)	$ (10,339,000)	$ (14,675,000)	$ (33,045,000)			$ (5,678,000)	$ (10,182,000)	$ (14,675,000)	$ (32,397,000)	$ (157,000)	$ (648,000)
Mark-to-market gains (losses)																							200,000	200,000
Unrealized gains (losses)																	(5,700,000)	(15,100,000)
Deferred gains (losses)																	(1,300,000)	(1,600,000)
Amount of (loss) gain recognized in OCI					(376,000)	(717,000)					(29,962,000)	(29,962,000)
Amount of (loss) gain reclassified from accumulated OCI into income (effective portion)			(194,000)	(592,000)			89,000	41,000	267,000	125,000
Cash flow hedge gain (loss) expected to be reclassed from accumulated OCI over the next 12 months									$ (500,000)
Natural gas requirements hedged [Abstract]
Millions MMBtus hedged for natural gas fuel costs	3.3	9.4
Estimated natural gas requirements covered (in hundredths)	4.00%	11.00%
Estimated natural gas requirements period (in years)	two year period	two-year period