Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Operating revenues		$ 1,080,534	$ 1,057,841
Net income		111,546	115,874
Affiliate electric revenue		2,000	2,000
Affiliate other revenue		1,000	1,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		711,480	717,543
Net income		76,144	72,536
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		356,192	328,539
Net income		34,483	40,890
All Other
Segment Reporting Information [Line Items]
Operating revenues		13,010	11,885
Net income		919	2,448
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	[1],[2]	1,080,534	1,057,841
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	[2]	711,388	717,462
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		356,136	328,494
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	[1]	13,010	11,885
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues		(148)	(126)
Net income		0	0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		92	81
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		56	45
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, 2017 and 2016.

[2] Operating revenues include $2 million of affiliate electric revenue for the three months ended March 31, 2017 and 2016.