Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2016		Sep. 30, 2015		Sep. 30, 2016		Sep. 30, 2015
Segment Reporting Information [Line Items]
Operating revenues	$ 1,059,177		$ 1,044,704		$ 3,026,870		$ 3,132,675
Net income (loss)	173,607		173,081		376,825		382,547
Affiliate electric revenue	2,000		2,000		7,000		6,000
Affiliate other revenue	1,000		2,000		3,000		3,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	897,570		884,370		2,337,743		2,385,519
Net income (loss)	168,328		167,931		320,192		330,276
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	152,769		151,562		659,822		716,774
Net income (loss)	4,918		4,548		53,883		51,784
All Other
Segment Reporting Information [Line Items]
Operating revenues	8,898		8,846		29,585		30,647
Net income (loss)	361		602		2,750		487
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	1,059,177	[1],[2]	1,044,704	[1],[2]	3,026,870	[3],[4]	3,132,675	[3],[4]
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	897,516	[1]	884,305	[1]	2,337,547	[4]	2,385,297	[4]
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	152,763		151,553		659,738		716,731
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	8,898	[2]	8,846	[2]	29,585	[3]	30,647	[3]
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(60)		(74)		(280)		(265)
Net income (loss)	0		0		0		0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	54		65		196		222
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	6		9		84		43
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0		$ 0		$ 0		$ 0

[1] (a) Operating revenues include $2 million of affiliate electric revenue for the three months ended Sept. 30, 2016 and 2015.

[2] (b) Operating revenues include $1 million and $2 million of other affiliate revenue for the three months ended Sept. 30, 2016 and 2015, respectively.

[3] Operating revenues include $3 million of other affiliate revenue for the nine months ended Sept. 30, 2016 and 2015.

[4] Operating revenues include $7 million and $6 million of affiliate electric revenue for the nine months ended Sept. 30, 2016 and 2015, respectively.