Statement of Comprehensive Income (Unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Statement of Comprehensive Income [Abstract]
OPERATING REVENUES	$ 114,644	$ 113,853	$ 233,729	$ 232,587
OPERATING EXPENSES:
General and administrative	15,842	15,773	31,444	31,983
Operation and maintenance	20,525	18,693	37,051	35,488
Depreciation	24,784	23,728	49,398	47,740
Regulatory debits	381	165	762	350
Taxes, other than income taxes	4,209	4,352	9,146	8,692
Total operating expenses	65,741	62,711	127,801	124,253
OPERATING INCOME	48,903	51,142	105,928	108,334
OTHER (INCOME) AND OTHER EXPENSES:
Interest expense	11,530	11,555	23,060	23,109
Allowance for equity and borrowed funds used during construction	(173)	(371)	(248)	(647)
Miscellaneous other (income) expenses, net	(110)	(67)	(23)	209
Total other (income) and other expenses	11,247	11,117	22,789	22,671
NET INCOME	37,656	40,025	83,139	85,663
CASH FLOW HEDGES:
Amortization of cash flow hedges into Interest expense	(16)	(16)	(31)	(31)
COMPREHENSIVE INCOME	$ 37,640	$ 40,009	$ 83,108	$ 85,632