Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Operating revenues		$ 1,073,298	$ 1,080,534
Net income		133,722	111,546
Affiliate electric revenue		1,000	2,000
Affiliate other revenue		1,000	1,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		698,386	711,480
Net income		79,551	76,144
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		364,050	356,192
Net income		53,712	34,483
All Other
Segment Reporting Information [Line Items]
Operating revenues		11,038	13,010
Net income		459	919
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	[1],[2]	1,073,298	1,080,534
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	[1]	698,274	711,388
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		363,986	356,136
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	[2]	11,038	13,010
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues		(176)	(148)
Net income		0	0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		112	92
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		64	56
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues		$ 0	$ 0

[1] Operating revenues include $1 million and $2 million of affiliate electric revenue for the three months ended March 31, 2018 and 2017.

[2] Operating revenues include $1 million of other affiliate revenue for the three months ended March 31, 2018 and 2017.