Segment and Related Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Operating revenues	$ 330,590	$ 300,998	$ 293,120	$ 332,539	$ 324,989	$ 272,739	$ 270,560	$ 346,011	$ 1,257,247	$ 1,214,299	$ 1,204,863
Cost of sales									400,973	372,864	482,591
Gross margin									856,274	841,435	722,272
Operating, general and administrative									302,893	297,475	305,886
Property and other taxes									148,098	133,442	114,592
Depreciation and depletion									159,336	144,702	123,776
Operating Income	64,156	56,116	63,742	61,933	72,332	48,461	61,132	83,891	245,947	265,816	178,018
Interest expense, net									(94,970)	(92,153)	(77,802)
Other income, net									5,548	7,583	10,198
Income tax (expense) benefit									7,647	(30,037)	10,272
Net Income	44,131	$ 44,605	$ 35,569	$ 39,867	45,013	$ 23,798	$ 30,973	$ 51,425	164,172	151,209	120,686
Total assets	5,499,321				5,264,695				5,499,321	5,264,695	4,960,902
Capital expenditures									287,901	283,705	270,384
Electric
Segment Reporting Information [Line Items]
Operating revenues									1,011,595	944,428	877,967
Cost of sales									332,817	281,251	348,640
Gross margin									678,778	663,177	529,327
Operating, general and administrative									216,736	233,416	200,186
Property and other taxes									115,583	104,264	84,759
Depreciation and depletion									130,236	115,701	94,813
Operating Income									216,223	209,796	149,569
Interest expense, net									(86,038)	(79,044)	(60,424)
Other income, net									3,246	6,300	4,758
Income tax (expense) benefit									7,392	(19,950)	(1,490)
Net Income									140,823	117,102	92,413
Total assets	4,363,848				4,185,192				4,363,848	4,185,192	3,434,035
Capital expenditures									236,014	234,451	233,538
Gas
Segment Reporting Information [Line Items]
Operating revenues									245,652	269,871	326,896
Cost of sales									68,156	91,613	133,951
Gross margin									177,496	178,258	192,945
Operating, general and administrative									86,713	84,219	91,437
Property and other taxes									32,505	29,168	29,821
Depreciation and depletion									29,067	28,968	28,930
Operating Income									29,211	35,903	42,757
Interest expense, net									(6,589)	(11,433)	(10,618)
Other income, net									1,329	1,821	1,324
Income tax (expense) benefit									(1,687)	(3,752)	(7,463)
Net Income									22,264	22,539	26,000
Total assets	1,129,355				1,072,613				1,129,355	1,072,613	1,519,196
Capital expenditures									51,887	49,254	36,846
Other Segments [Member]
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									(556)	(20,160)	14,263
Property and other taxes									10	10	12
Depreciation and depletion									33	33	33
Operating Income									513	20,117	(14,308)
Interest expense, net									(2,343)	(1,676)	(6,760)
Other income, net									973	(538)	4,116
Income tax (expense) benefit									1,942	(6,335)	19,225
Net Income									1,085	11,568	2,273
Total assets	6,118				6,890				6,118	6,890	7,671
Capital 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
Property and other taxes									0	0	0
Depreciation and depletion									0	0	0
Operating Income									0	0	0
Interest expense, net									0	0	0
Other income, net									0	0	0
Income tax (expense) benefit									0	0	0
Net Income									0	0	0
Total assets	$ 0				$ 0				0	0	0
Capital expenditures									$ 0	$ 0	$ 0