Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2015		Sep. 30, 2014		Sep. 30, 2015		Sep. 30, 2014
Segment Reporting Information [Line Items]
Operating revenues	$ 1,044,704		$ 1,049,111		$ 3,132,675		$ 3,246,358
Net income (loss)	173,081		154,159		382,547		362,354
Affiliate electric revenue	2,000		2,000		6,000		8,000
Affiliate other revenue	2,000		1,000		3,000		3,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	884,370		881,197		2,385,519		2,377,189
Net income (loss)	167,931		139,751		330,276		289,460
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	151,562		159,837		716,774		839,469
Net income (loss)	4,548		7,235		51,784		56,230
All Other
Segment Reporting Information [Line Items]
Operating revenues	8,846		8,201		30,647		30,091
Net income (loss)	602		7,173		487		16,664
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	1,044,704	[1],[2]	1,049,111	[1],[2]	3,132,675	[3],[4]	3,246,358	[3],[4]
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	884,305	[2]	881,102	[2]	2,385,297	[4]	2,376,935	[4]
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	151,553		159,808		716,731		839,332
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	8,846	[1]	8,201	[1]	30,647	[3]	30,091	[3]
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(74)		(124)		(265)		(391)
Net income (loss)	0		0		0		0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	65		95		222		254
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	9		29		43		137
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0		$ 0		$ 0		$ 0

[1] Operating revenues include $2 million and $1 million of other affiliate revenue for the three months ended Sept. 30, 2015 and 2014, respectively.

[2] Operating revenues include $2 million of affiliate electric revenue for the three months ended Sept. 30, 2015 and 2014.

[3] Operating revenues include $3 million of other affiliate revenue for the nine months ended Sept. 30, 2015 and 2014.

[4] Operating revenues include $6 million and $8 million of affiliate electric revenue for the nine months ended Sept. 30, 2015 and 2014