Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Segment Reporting Information [Line Items]
Operating revenues		$ 1,234,633	$ 1,292,482
Net income (loss)		94,629	6,924	[1]
Affiliate electric revenue		124,896	124,875
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		1,033,788	1,006,287
Net income (loss)		71,321	(29,599)	[1]
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		194,292	279,877
Net income (loss)		23,142	40,272
All Other
Segment Reporting Information [Line Items]
Operating revenues		6,860	6,861
Net income (loss)		166	(3,749)
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	[2],[3]	1,234,633	1,292,482
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	[2]	1,033,643	1,006,154
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	[3]	194,130	279,467
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues		6,860	6,861
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues		(307)	(543)
Net income (loss)		0	0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		145	133
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		162	410
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues		$ 0	$ 0

[1] Includes a net of tax charge related to the Monticello LCM/EPU project. See Note 5.

[2] Operating revenues include $125 million of affiliate electric revenue for the three months ended March 31, 2016 and 2015.

[3] Operating revenues include an immaterial amount of affiliate gas revenue for the three months ended March 31, 2016 and 2015.