CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
OPERATING ACTIVITIES:
Net Income	$ 98,406	$ 92,556	$ 77,376
Items not affecting cash:
Depreciation	106,044	100,926	91,769
Amortization of debt issue costs, discount and deferred hedge gain	369	1,032	1,827
Amortization of nonvested shares	2,759	2,133	1,622
Equity portion of allowance for funds used during construction	(4,846)	(1,877)	(6,564)
(Gain) loss on disposition of assets	(332)	811	11
Deferred income taxes	51,890	64,065	48,783
Mountain States Transmission Intertie impairment	24,039	0	0
Gain on CELP arbitration decision	(47,894)	0	0
Changes in current assets and liabilities:
Restricted cash	6,016	146	746
Accounts receivable	3,456	(3,847)	455
Inventories	5,371	(8,831)	(3,396)
Other current assets	(1,856)	(3,551)	8,155
Accounts payable	10,976	(1,928)	(12,766)
Accrued expenses	14,149	1,883	31,064
Regulatory assets	(6,285)	1,684	(13,575)
Regulatory liabilities	15,241	16,020	(12,449)
Other noncurrent assets	(27,362)	(30,048)	5,332
Other noncurrent liabilities	1,052	2,583	530
Cash provided by operating activities	251,193	233,757	218,920
INVESTING ACTIVITIES:
Property, plant, and equipment additions	(219,234)	(188,730)	(228,373)
Asset acquisitions	(103,241)	0	(12,372)
Proceeds from sale of assets	262	209	69
Cash used in investing activities	(322,213)	(188,521)	(240,676)
FINANCING ACTIVITIES:
Dividends on common stock	(54,246)	(51,909)	(48,997)
Proceeds from issuance of common stock, net	28,477	0	0
Issuance of long-term debt	150,000	0	225,000
Repayment of long-term debt	(3,945)	(6,589)	(231,152)
Line of credit borrowings	0	80,000	695,000
Line of credit repayments	0	(233,000)	(608,000)
(Repayments) issuances of short-term borrowings, net	(44,000)	166,934	0
Treasury stock activity	(429)	153	(185)
Financing costs	(943)	(1,131)	(8,020)
Cash provided by (used in) financing activities	74,914	(45,542)	23,646
Increase (Decrease) in Cash and Cash Equivalents	3,894	(306)	1,890
Cash and Cash Equivalents, beginning of period	5,928	6,234	4,344
Cash and Cash Equivalents, end of period	$ 9,822	$ 5,928	$ 6,234