Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
Operating revenues	$ 260,161	$ 244,603	$ 573,181	$ 553,703
Cost of sales	106,913	96,427	239,109	234,823
Gross margin	153,248	148,176	334,072	318,880
Operating, general and administrative	67,364	67,096	136,201	132,669
Property and other taxes	25,810	25,934	51,569	49,599
Depreciation	27,414	26,426	56,632	52,859
Operating income (loss)	32,660	28,720	89,670	83,753
Interest expense	(17,141)	(15,893)	(33,920)	(31,855)
Other income	928	1,176	3,643	2,160
Income tax (expense) benefit	(2,106)	(2,565)	(7,150)	(10,577)
Net Income (Loss)	14,341	11,438	52,243	43,481
Total assets	3,551,048	3,224,089	3,551,048	3,224,089	3,485,533
Capital expenditures	50,435	50,705	88,549	97,812
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	200,472	196,176	410,564	403,231
Cost of sales	82,520	78,109	165,615	161,088
Gross margin	117,952	118,067	244,949	242,143
Operating, general and administrative	47,721	47,685	93,439	93,042
Property and other taxes	19,016	19,469	38,168	37,007
Depreciation	21,693	21,565	45,304	43,134
Operating income (loss)	29,522	29,348	68,038	68,960
Interest expense	(14,411)	(13,409)	(28,538)	(27,076)
Other income	702	801	2,713	1,413
Income tax (expense) benefit	(76)	(5,910)	(4,380)	(9,084)
Net Income (Loss)	15,737	10,830	37,833	34,213
Total assets	2,486,588	2,268,913	2,486,588	2,268,913
Capital expenditures	41,691	44,712	75,006	84,791
Natural Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	59,362	48,101	161,880	149,847
Cost of sales	24,393	18,318	73,494	73,735
Gross margin	34,969	29,783	88,386	76,112
Operating, general and administrative	18,483	18,657	38,378	37,945
Property and other taxes	6,792	6,463	13,396	12,587
Depreciation	5,712	4,853	11,311	9,709
Operating income (loss)	3,982	(190)	25,301	15,871
Interest expense	(2,567)	(2,230)	(4,993)	(4,297)
Other income	198	349	875	694
Income tax (expense) benefit	(1,901)	1,010	(3,673)	(2,580)
Net Income (Loss)	(288)	(1,061)	17,510	9,688
Total assets	1,054,550	943,471	1,054,550	943,471
Capital expenditures	8,744	5,993	13,543	13,021
All Other Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	327	326	737	625
Cost of sales	0	0	0	0
Gross margin	327	326	737	625
Operating, general and administrative	1,160	754	4,384	1,682
Property and other taxes	2	2	5	5
Depreciation	9	8	17	16
Operating income (loss)	(844)	(438)	(3,669)	(1,078)
Interest expense	(163)	(254)	(389)	(482)
Other income	28	26	55	53
Income tax (expense) benefit	(129)	2,335	903	1,087
Net Income (Loss)	(1,108)	1,669	(3,100)	(420)
Total assets	9,910	11,705	9,910	11,705
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
Property and other taxes	0	0	0	0
Depreciation	0	0	0	0
Operating income (loss)	0	0	0	0
Interest expense	0	0	0	0
Other income	0	0	0	0
Income tax (expense) benefit	0	0	0	0
Net Income (Loss)	0	0	0	0
Total assets	0	0	0	0
Capital expenditures	$ 0	$ 0	$ 0	$ 0