Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016
Segment Reporting Information [Line Items]
Operating revenues	$ 1,030,293		$ 1,059,177		$ 3,041,743		$ 3,026,870
Net income	186,077		173,607		398,210		376,825
Affiliate electric revenue	0		2,000		1,000		7,000
Affiliate other revenue	1,000		1,000		3,000		3,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	877,651		897,570		2,319,118		2,337,743
Net income	178,648		168,328		342,195		320,192
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	142,611		152,769		691,620		659,822
Net income	5,815		4,918		53,133		53,883
All Other
Segment Reporting Information [Line Items]
Operating revenues	10,300		8,898		31,529		29,585
Net income	1,614		361		2,882		2,750
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	1,030,293	[1],[2]	1,059,177	[1],[2]	3,041,743	[3],[4]	3,026,870	[3],[4]
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	877,604	[1]	897,516	[1]	2,318,912	[3]	2,337,547	[3]
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	142,389		152,763		691,302		659,738
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	10,300	[2]	8,898	[2]	31,529	[4]	29,585	[4]
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(269)		(60)		(524)		(280)
Net income	0		0		0		0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	47		54		206		196
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	222		6		318		84
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0		$ 0		$ 0		$ 0

[1] Operating revenues include $0 and $2 million of affiliate electric revenue for the three months ended Sept. 30, 2017 and 2016.

[2] Operating revenues include $1 million of other affiliate revenue for the three months ended Sept. 30, 2017 and 2016.

[3] Operating revenues include $1 million and $7 million of affiliate electric revenue for the nine months ended Sept. 30, 2017 and 2016, respectively.

[4] Operating revenues include $3 million of other affiliate revenue for the nine months ended Sept. 30, 2017 and 2016.