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]
Intercompany Revenue										$ 13,000	$ 14,000	$ 13,000
Operating revenues		$ 1,030,838	$ 1,044,704	$ 952,521	$ 1,135,450	$ 1,136,791	$ 1,049,111	$ 993,704	$ 1,203,543	4,163,513	4,383,149	4,202,628
Depreciation and amortization										411,667	379,202	360,417
Total interest charges and financing costs										171,908	154,640	160,945
Income tax expense (benefit)										278,440	243,591	250,740
Net income (loss)		$ 84,255	$ 173,081	$ 98,500	$ 110,966	$ 92,834	$ 154,159	$ 89,792	$ 118,403	466,802	455,188	453,383
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues										3,115,558	3,126,276	3,081,473
Depreciation and amortization										311,122	285,968	280,972
Total interest charges and financing costs										136,397	124,118	129,787
Income tax expense (benefit)										234,873	208,095	220,356
Net income (loss)										391,257	349,793	368,586
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues										1,006,733	1,215,504	1,080,813
Depreciation and amortization										96,384	89,186	75,510
Total interest charges and financing costs										34,935	29,987	30,604
Income tax expense (benefit)										44,192	50,874	42,294
Net income (loss)										74,267	84,324	69,682
All Other
Segment Reporting Information [Line Items]
Operating revenues										41,590	41,888	40,754
Depreciation and amortization										4,161	4,048	3,935
Total interest charges and financing costs										576	535	554
Income tax expense (benefit)										(625)	(15,378)	(11,910)
Net income (loss)										1,278	21,071	15,115
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	[1]									4,163,513	4,383,149	4,202,628
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	[1]									3,115,257	3,125,937	3,081,171
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	[1]									1,006,666	1,215,324	1,080,703
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	[1]									41,590	41,888	40,754
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues										(368)	(519)	(412)
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										301	339	302
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues										67	180	110
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues										$ 0	$ 0	$ 0

[1] Operating revenues include $13 million, $14 million and $13 million of intercompany revenue for the years ended Dec. 31, 2015, 2014 and 2013, respectively. See Note 16 for further discussion of related party transactions by reportable segment.