Statement of Comprehensive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
OPERATING REVENUES	$ 470,050	$ 469,661	$ 437,835
OPERATING EXPENSES:
General and administrative	62,085	62,473	68,997
Operation and maintenance	74,191	75,342	77,580
Depreciation	99,138	96,109	93,419
Regulatory debits (credits)	1,487	921	(494)
Taxes, other than income taxes	16,987	16,882	16,751
Total operating expenses	253,888	251,727	256,253
OPERATING INCOME	216,162	217,934	181,582
OTHER (INCOME) AND OTHER EXPENSES:
Interest expense	46,095	46,177	46,357
Allowance for equity and borrowed funds used during construction	(573)	(1,405)	(2,291)
Miscellaneous other expenses, net	87	175	793
Total other (income) and other expenses	45,609	44,947	44,859
NET INCOME	170,553	172,987	136,723
CASH FLOW HEDGES:
Amortization of cash flow hedges into Interest expense	(62)	(62)	(62)
COMPREHENSIVE INCOME	$ 170,491	$ 172,925	$ 136,661