Statement of Comprehensive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Statement of Comprehensive Income [Abstract]
OPERATING REVENUES	$ 469,661	$ 437,835	$ 434,484
OPERATING EXPENSES:
General and administrative	62,473	68,997	56,226
Operation and maintenance	75,342	77,580	74,911
Depreciation	96,109	93,419	90,486
Regulatory debits (credits)	921	(494)	(1,023)
Taxes, other than income taxes	16,882	16,751	16,655
Total operating expenses	251,727	256,253	237,255
OPERATING INCOME	217,934	181,582	197,229
OTHER (INCOME) AND OTHER EXPENSES:
Interest expense	46,177	46,357	46,433
Allowance for equity and borrowed funds used during construction	(1,405)	(2,291)	(2,101)
Miscellaneous other expenses, net	175	793	84
Total other (income) and other expenses	44,947	44,859	44,416
NET INCOME	172,987	136,723	152,813
CASH FLOW HEDGES:
Amortization of cash flow hedges into Interest expense	(62)	(62)	(62)
COMPREHENSIVE INCOME	$ 172,925	$ 136,661	$ 152,751