Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Segment Reporting Information [Line Items]
Operating revenues		$ 1,057,841	$ 1,135,450
Net income (loss)		115,874	110,966
Affiliate electric revenue		2,000	2,000
Affiliate other revenue		1,000	1,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		717,543	749,549
Net income (loss)		72,536	74,669
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		328,539	373,244
Net income (loss)		40,890	36,623
All Other
Segment Reporting Information [Line Items]
Operating revenues		11,885	12,786
Net income (loss)		2,448	(326)
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	[1],[2]	1,057,841	1,135,450
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	[2]	717,462	749,461
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		328,494	373,203
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	[1]	11,885	12,786
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues		(126)	(129)
Net income (loss)		0	0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		81	88
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		45	41
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues		$ 0	$ 0

[1] Operating revenues include $1 million of other affiliate revenue for the three months ended March 31, 2016 and 2015.

[2] Operating revenues include $2 million of affiliate electric revenue for the three months ended March 31, 2016 and 2015