QUARTERLY RESULTS OF OPERATIONS - UNAUDITED (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Information Disclosure [Abstract]
Operating Revenues	$ 129,010	$ 54,251	$ 59,177	$ 179,436	$ 108,457	$ 58,482	$ 65,187	$ 180,323	$ 421,874	$ 412,449	$ 475,982
Expenses:
Cost of Sales (Excluding depreciation)	60,751	21,053	20,407	86,499	43,517	27,516	28,259	88,574	188,710	187,866	266,382
Operations and Maintenance Including Fixed Charges	33,877	32,953	33,597	35,102	32,483	31,261	33,036	33,771
Income Taxes (Benefit)	10,589	258	1,887	20,977	11,869	(699)	1,065	22,046	33,711	34,281	29,678
Energy and Other Taxes	2,485	1,167	1,479	3,169	2,671	1,384	1,722	4,514	8,300	10,291	10,769
Total Expenses	107,702	55,431	57,370	145,747	90,540	59,462	64,082	148,905
Other Income and Expense	(1,865)	1,703	1,430	1,349	1,386	650	515	878
Net Income (Loss) Applicable to Common Stock	$ 19,443	$ 523	$ 3,237	$ 35,038	$ 19,303	$ (330)	$ 1,620	$ 32,296	$ 58,241	$ 52,889	$ 43,925