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]
Equity investments in unconsolidated subsidiaries	$ 132,800				$ 130,000				$ 132,800	$ 130,000
Operating revenues	2,794,651	$ 3,040,147	$ 2,499,849	$ 2,772,273	2,645,821	$ 2,901,312	$ 2,515,134	$ 2,962,219	11,106,920	11,024,486	$ 11,686,135
Depreciation and amortization									1,303,203	1,124,524	1,019,045
Total interest charges and financing costs									619,879	569,034	528,206
Income tax expense (benefit)									581,217	542,719	523,815
Net income (loss)	$ 227,477	$ 457,795	$ 196,795	$ 241,312	$ 209,025	$ 426,463	$ 196,931	$ 152,066	1,123,379	984,485	1,021,306
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues									9,501,108	9,277,497	9,467,664
Depreciation and amortization									1,135,584	962,565	866,746
Total interest charges and financing costs									449,916	425,999	397,824
Income tax expense (benefit)									566,957	508,568	512,551
Net income (loss)									1,066,758	921,403	890,535
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues									1,532,522	1,673,332	2,148,631
Depreciation and amortization									160,293	154,892	144,661
Total interest charges and financing costs									53,913	49,763	43,940
Income tax expense (benefit)									76,378	60,545	76,418
Net income (loss)									124,250	106,023	128,559
All Other
Segment Reporting Information [Line Items]
Operating revenues									75,727	76,419	77,507
Depreciation and amortization									7,326	7,067	7,638
Total interest charges and financing costs									116,050	93,272	86,442
Income tax expense (benefit)									(62,118)	(26,394)	(65,154)
Net income (loss)									(67,629)	(42,941)	2,212
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues									11,106,920	11,024,486	11,686,135
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues									9,499,781	9,275,986	9,465,890
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues									1,531,412	1,672,081	2,142,738
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues									75,727	76,419	77,507
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues									(2,437)	(2,762)	(7,667)
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									1,327	1,511	1,774
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues									1,110	1,251	5,893
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues									$ 0	$ 0	$ 0