Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Segment Reporting Information [Line Items]
Operating revenues	$ 235,866	$ 244,041	$ 789,569	$ 834,107
Cost of sales	93,061	98,045	327,884	370,523
Gross margin	142,805	145,996	461,685	463,584
Operating, general and administrative	63,056	66,332	195,725	203,254
Mountain States Transmission Intertie impairment	24,039	0	24,039	0
Property and other taxes	24,796	22,605	74,395	68,551
Depreciation	26,505	25,181	79,364	75,562
Operating Income	4,409	31,878	88,162	116,217
Interest expense	(17,743)	(16,694)	(49,598)	(50,737)
Other income	974	346	3,134	2,257
Income tax expense	8,588	(635)	(1,989)	(9,297)
Net (Loss) Income	(3,772)	14,895	39,709	58,440
Total assets	3,306,946	3,061,824	3,306,946	3,061,824	3,210,438
Capital expenditures	78,431	52,627	176,243	124,484
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	202,485	206,613	605,716	602,024
Cost of sales	83,814	85,221	244,902	246,592
Gross margin	118,671	121,392	360,814	355,432
Operating, general and administrative	44,711	45,607	137,753	140,267
Mountain States Transmission Intertie impairment	24,039		24,039
Property and other taxes	18,621	16,894	55,628	50,937
Depreciation	21,636	20,465	64,770	61,205
Operating Income	9,664	38,426	78,624	103,023
Interest expense	(15,181)	(13,661)	(42,257)	(40,877)
Other income	405	86	1,818	1,425
Income tax expense	5,762	(3,407)	(3,322)	(10,998)
Net (Loss) Income	650	21,444	34,863	52,573
Total assets	2,319,856	2,157,225	2,319,856	2,157,225
Capital expenditures	45,932	44,318	130,723	99,168
Natural Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	32,965	37,067	182,812	230,971
Cost of sales	9,247	12,824	82,982	123,931
Gross margin	23,718	24,243	99,830	107,040
Operating, general and administrative	17,452	19,979	55,397	60,651
Mountain States Transmission Intertie impairment	0		0
Property and other taxes	6,172	5,708	18,759	17,606
Depreciation	4,860	4,708	14,569	14,332
Operating Income	(4,766)	(6,152)	11,105	14,451
Interest expense	(2,363)	(2,711)	(6,660)	(8,105)
Other income	541	232	1,235	751
Income tax expense	3,102	3,016	522	(1,228)
Net (Loss) Income	(3,486)	(5,615)	6,202	5,869
Total assets	976,152	891,989	976,152	891,989
Capital expenditures	32,499	8,309	45,520	25,316
All Other Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	416	361	1,041	1,112
Cost of sales	0	0	0	0
Gross margin	416	361	1,041	1,112
Operating, general and administrative	893	746	2,575	2,336
Mountain States Transmission Intertie impairment	0		0
Property and other taxes	3	3	8	8
Depreciation	9	8	25	25
Operating Income	(489)	(396)	(1,567)	(1,257)
Interest expense	(199)	(322)	(681)	(1,755)
Other income	28	28	81	81
Income tax expense	(276)	(244)	811	2,929
Net (Loss) Income	(936)	(934)	(1,356)	(2)
Total assets	10,938	12,610	10,938	12,610
Capital expenditures	0	0	0	0
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Cost of sales	0	0	0	0
Gross margin	0	0	0	0
Operating, general and administrative	0	0	0	0
Mountain States Transmission Intertie impairment	0		0
Property and other taxes	0	0	0	0
Depreciation	0	0	0	0
Operating Income	0	0	0	0
Interest expense	0	0	0	0
Other income	0	0	0	0
Income tax expense	0	0	0	0
Net (Loss) Income	0	0	0	0
Total assets	0	0	0	0
Capital expenditures	$ 0	$ 0	$ 0	$ 0