RECLASSIFICATIONS FROM ACCUMULATED OTHER COMPREHENSIVE INCOME BY COMPONENT Reclassifications From Accumulated Other Comprehensive Income By Component) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax [Abstract]
Interest Expense Long-Term Debt	$ 15,579	$ 13,848	$ 45,326	$ 42,412
Interest Expense Capital Leases	1,202	6,323	9,048	18,821
Purchased Energy	49,902	42,477	125,423	89,815
Other Nonoperating Expense	(7,170)	(2,776)	(11,979)	(7,493)
Income Tax Benefit	(23,576)	(38,828)	(51,656)	(41,737)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(674)	(885)	(1,914)	(2,078)
Other Comprehensive Income (Loss), Amortization Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Prior Service (Cost) Credit, Net of Tax	25	68	74	205
Total Reclassification from Other Comprehensive Income for the Period	(699)	(953)	(1,988)	(2,283)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Realized Losses on Cash Flow Hedges, Net of Taxes
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax [Abstract]
Income Tax Benefit	546	579	1,238	1,360
Reclassification out of Accumulated Other Comprehensive Income [Member] | Realized Losses on Cash Flow Hedges, Net of Taxes | Interest Rate Swaps - Debt
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax [Abstract]
Interest Expense Long-Term Debt	(291)	(296)	(882)	(871)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Realized Losses on Cash Flow Hedges, Net of Taxes | Interest Rate Swaps - Capital Leases
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax [Abstract]
Interest Expense Capital Leases	(451)	(612)	(1,649)	(1,820)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Realized Losses on Cash Flow Hedges, Net of Taxes | Commodity Contracts
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax [Abstract]
Purchased Energy	(478)	(556)	(621)	(747)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Amortization, Net of Taxes
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax [Abstract]
Other Nonoperating Expense	(40)	(110)	(119)	(332)
Income Tax Benefit	$ 15	$ 42	$ 45	$ 127