QUARTERLY RESULTS OF OPERATIONS - UNAUDITED (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Operating Revenues	$ 142,502	$ 62,025	$ 68,762	$ 187,766	$ 132,186	$ 58,634	$ 75,812	$ 267,658	$ 461,055	$ 534,290	$ 501,875
Expenses:
Cost of Sales (excluding depreciation)	58,695	26,395	19,997	69,303	50,835	22,934	25,419	146,102	174,390	245,290	231,216
Operations and Maintenance Including Fixed Charges	47,886	41,303	42,826	46,450	48,258	42,204	42,691	52,137
Income Taxes	12,554	(2,007)	1,415	27,404	10,352	(2,871)	3,292	26,172	39,366	36,945	34,895
Energy and Other Taxes	1,195	838	560	1,027	951	806	854	1,420	3,620	4,031	3,760
Total Expenses	120,330	66,529	64,798	144,184	110,396	63,073	72,256	225,831
Other Income and Expense	727	1,189	1,079	836	404	1,010	1,670	760
Net Income	$ 22,899	$ (3,315)	$ 5,043	$ 44,418	$ 22,194	$ (3,429)	$ 5,226	$ 42,587	$ 69,045	$ 66,578	$ 66,483