Segment and Related Information (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
Segment Reporting Information [Line Items]
Operating revenues	$ 280,773	$ 235,866	$ 244,603	$ 309,100	$ 283,209	$ 244,041	$ 251,806	$ 338,260	$ 1,070,342	$ 1,117,316	$ 1,110,720
Cost of sales									395,434	494,559	531,089
Gross margin									674,908	622,757	579,631
Operating, general and administrative									269,966	267,160	237,047
MSTI impairment									24,039	0	0
Property and other taxes									97,674	89,122	88,198
Depreciation									106,044	100,926	91,769
Operating Income	89,024	4,409	28,720	55,033	49,332	31,878	26,244	58,095	177,185	165,549	162,617
Interest expense									(65,062)	(66,859)	(65,826)
Other income									4,372	3,931	6,345
Income tax (expense) benefit									(18,089)	(10,065)	(25,760)
Net Income	58,697	(3,772)	11,438	32,043	34,116	14,895	10,970	32,575	98,406	92,556	77,376
Total assets	3,485,533				3,210,438				3,485,533	3,210,438	3,037,669
Captial expenditures									219,234	188,730	228,373
Electric
Segment Reporting Information [Line Items]
Operating revenues									805,554	797,562	790,701
Cost of sales									277,826	327,126	356,325
Gross margin									527,728	470,436	434,376
Operating, general and administrative									187,599	183,503	169,483
MSTI impairment									24,039
Property and other taxes									72,755	66,425	65,027
Depreciation									86,559	81,859	74,227
Operating Income									156,776	138,649	125,639
Interest expense									(55,118)	(54,394)	(49,576)
Other income									2,630	2,563	5,954
Income tax (expense) benefit									(22,298)	(14,049)	(18,939)
Net Income									81,990	72,769	63,078
Total assets	2,442,602				2,259,189				2,442,602	2,259,189	2,136,784
Captial expenditures									178,325	146,576	187,212
Gas
Segment Reporting Information [Line Items]
Operating revenues									263,394	318,335	318,735
Cost of sales									117,608	167,433	174,764
Gross margin									145,786	150,902	143,971
Operating, general and administrative									75,971	80,431	71,088
MSTI impairment									0
Property and other taxes									24,907	22,686	23,159
Depreciation									19,452	19,034	17,509
Operating Income									25,456	28,751	32,215
Interest expense									(9,063)	(10,432)	(12,608)
Other income									1,633	1,258	284
Income tax (expense) benefit									(692)	(3,472)	(4,183)
Net Income									17,334	16,105	15,708
Total assets	1,032,259				938,876				1,032,259	938,876	887,799
Captial expenditures									40,909	42,154	41,161
Other
Segment Reporting Information [Line Items]
Operating revenues									1,394	1,419	1,284
Cost of sales									0	0	0
Gross margin									1,394	1,419	1,284
Operating, general and administrative									6,396	3,226	(3,524)
MSTI impairment									0
Property and other taxes									12	11	12
Depreciation									33	33	33
Operating Income									(5,047)	(1,851)	4,763
Interest expense									(881)	(2,033)	(3,642)
Other income									109	110	107
Income tax (expense) benefit									4,901	7,456	(2,638)
Net Income									(918)	3,682	(1,410)
Total assets	10,672				12,373				10,672	12,373	13,086
Captial expenditures									0	0	0
Eliminations
Segment Reporting Information [Line Items]
Operating revenues									0	0	0
Cost of sales									0	0	0
Gross margin									0	0	0
Operating, general and administrative									0	0	0
MSTI impairment									0
Property and other taxes									0	0	0
Depreciation									0	0	0
Operating Income									0	0	0
Interest expense									0	0	0
Other income									0	0	0
Income tax (expense) benefit									0	0	0
Net Income									0	0	0
Total assets	0				0				0	0	0
Captial expenditures									$ 0	$ 0	$ 0