QUARTERLY RESULTS OF OPERATIONS - UNAUDITED (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Operating Revenues	$ 161,219	$ 60,952	$ 69,159	$ 210,545	$ 146,172	$ 59,674	$ 66,536	$ 174,098	$ 501,875	$ 446,480	$ 421,874
Expenses:
Cost of Sales (excluding depreciation)	79,644	23,400	24,879	103,293	71,654	24,717	26,108	77,602	231,216	200,081	188,710
Operations and Maintenance Including Fixed Charges	47,943	37,432	38,737	46,969	39,486	33,249	33,871	38,659
Income Taxes	9,455	534	2,379	22,527	11,322	471	2,269	20,771	34,895	34,833	33,711
Energy and Other Taxes	947	798	830	1,185	2,530	1,052	1,277	3,003	3,760	7,862	8,300
Total Expenses	137,989	62,164	66,825	173,974	124,992	59,489	63,525	140,035
Other Income and Expense	811	2,186	1,477	1,086	773	764	810	1,450
Net Income	$ 24,041	$ 974	$ 3,811	$ 37,657	$ 21,953	$ 949	$ 3,821	$ 35,513	$ 66,483	$ 62,236	$ 58,241