Segments 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]
Intercompany Revenue										$ 13,000	$ 13,000	$ 14,000
Operating revenues		$ 1,020,926	$ 1,059,177	$ 909,852	$ 1,057,841	$ 1,030,838	$ 1,044,704	$ 952,521	$ 1,135,450	4,047,796	4,163,513	4,383,149
Depreciation and amortization										443,555	411,667	379,202
Total interest charges and financing costs										174,586	171,908	154,640
Income tax expense (benefit)										273,918	278,440	243,591
Net income (loss)		$ 86,666	$ 173,607	$ 87,344	$ 115,874	$ 84,255	$ 173,081	$ 98,500	$ 110,966	463,491	466,802	455,188
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues										3,049,627	3,115,558	3,126,276
Depreciation and amortization										337,583	311,122	285,968
Total interest charges and financing costs										136,274	136,397	124,118
Income tax expense (benefit)										228,825	234,873	208,095
Net income (loss)										383,973	391,257	349,793
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues										957,831	1,006,733	1,215,504
Depreciation and amortization										101,663	96,384	89,186
Total interest charges and financing costs										37,881	34,935	29,987
Income tax expense (benefit)										45,960	44,192	50,874
Net income (loss)										75,426	74,267	84,324
All Other
Segment Reporting Information [Line Items]
Operating revenues										40,723	41,590	41,888
Depreciation and amortization										4,309	4,161	4,048
Total interest charges and financing costs										431	576	535
Income tax expense (benefit)										(867)	(625)	(15,378)
Net income (loss)										4,092	1,278	21,071
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	[1]									4,047,796	4,163,513	4,383,149
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	[1]									3,049,352	3,115,257	3,125,937
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	[1]									957,721	1,006,666	1,215,324
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	[1]									40,723	41,590	41,888
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues										(385)	(368)	(519)
Depreciation and amortization										0	0	0
Total interest charges and financing costs										0	0	0
Income tax expense (benefit)										0	0	0
Net income (loss)										0	0	0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues										275	301	339
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues										110	67	180
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues										$ 0	$ 0	$ 0

[1] Operating revenues include $13 million, $13 million and $14 million of intercompany revenue for the years ended Dec. 31, 2016, 2015 and 2014, respectively. See Note 16 for further discussion of related party transactions by reportable segment.