CONSOLIDATED STATEMENTS OF INCOME (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues
Electric	$ 877,967	$ 865,239	$ 805,554
Gas	326,896	287,605	263,394
Other	0	1,675	1,394
Total Revenues	1,204,863	1,154,519	1,070,342
Operating Expenses
Cost of sales	482,591	479,546	395,434
Operating, general and administrative	305,886	285,569	269,966
Mountain States Transmission Intertie impairment	0	0	24,039
Property and other taxes	114,592	105,540	97,674
Depreciation and depletion	123,776	112,831	106,044
Total Operating Expenses	1,026,845	983,486	893,157
Operating Income	178,018	171,033	177,185
Interest Expense	(77,802)	(70,486)	(65,062)
Other Income	10,198	7,737	4,372
Income Before Income Taxes	110,414	108,284	116,495
Income Tax Benefit (Expense)	10,272	(14,301)	(18,089)
Net Income	$ 120,686	$ 93,983	$ 98,406
Average Common Shares Outstanding	40,156,177	38,144,852	36,847,427
Basic Earnings per Average Common Share	$ 3.01	$ 2.46	$ 2.67
Diluted Earnings per Average Common Share	$ 2.99	$ 2.46	$ 2.66