Segments and Related Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Equity investments in unconsolidated subsidiaries	$ 130,000				$ 83,100				$ 130,000	$ 83,100
Operating revenues	2,645,821	$ 2,901,312	$ 2,515,134	$ 2,962,219	2,928,628	$ 2,869,807	$ 2,685,096	$ 3,202,604	11,024,486	11,686,135	$ 10,914,922
Depreciation and amortization									1,124,524	1,019,045	977,863
Total interest charges and financing costs									569,034	528,206	536,020
Income tax expense (benefit)									542,719	523,815	483,976
Net income (loss)	209,025	$ 426,463	$ 196,931	$ 152,066	196,339	$ 368,582	$ 195,164	$ 261,221	984,485	1,021,306	948,234
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues									9,277,497	9,467,664	9,035,377
Depreciation and amortization									962,565	866,746	840,833
Total interest charges and financing costs									425,999	397,824	386,198
Income tax expense (benefit)									508,568	512,551	495,044
Net income (loss)									921,403	890,535	850,572
Regulated Natural Gas
Segment Reporting Information [Line Items]
Equity investments in unconsolidated subsidiaries	$ 130,000				$ 83,100				130,000	83,100
Operating revenues									1,673,332	2,148,631	1,807,396
Depreciation and amortization									154,892	144,661	128,186
Total interest charges and financing costs									49,763	43,940	44,927
Income tax expense (benefit)									60,545	76,418	25,543
Net income (loss)									106,023	128,559	123,702
All Other
Segment Reporting Information [Line Items]
Operating revenues									76,419	77,507	76,198
Depreciation and amortization									7,067	7,638	8,844
Total interest charges and financing costs									93,272	86,442	104,895
Income tax expense (benefit)									(26,394)	(65,154)	(36,611)
Net income (loss)									(42,941)	2,212	(26,040)
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues									11,024,486	11,686,135	10,914,922
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues									9,275,986	9,465,890	9,034,045
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues									1,672,081	2,142,738	1,804,679
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues									76,419	77,507	76,198
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues									(2,762)	(7,667)	(4,049)
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									1,511	1,774	1,332
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues									1,251	5,893	2,717
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues									$ 0	$ 0	$ 0