Revenues (Tables)

3 Months Ended

Mar. 31, 2018

Revenue from Contract with Customer [Abstract]

Disaggregation of Revenue [Table Text Block]

	Three Months Ended March 31, 2018
(Thousands of Dollars)	Regulated Electric		Regulated Natural Gas		All Other		Total
Major revenue types
Revenue from contracts with customers:
Residential	$	67,803	$	30,326	$	16	$	98,145
Commercial and industrial (C&I)	104,817		24,550		25		129,392
Other	1,606		—		237		1,843
Total retail	174,226		54,876		278		229,380
Interchange	37,674		—		—		37,674
Other	1,504		1,007		—		2,511
Total revenue from contracts with customers	213,404		55,883		278		269,565
Alternative revenue and other	2,962		617		—		3,579
Total revenues	$	216,366	$	56,500	$	278	$	273,144

	Three Months Ended March 31, 2017
(Thousands of Dollars)	Regulated Electric		Regulated Natural Gas		All Other		Total
Major revenue types
Revenue from contracts with customers:
Residential	$	66,043	$	26,628	$	20	$	92,691
C&I	102,380		20,258		24		122,662
Other	1,520		—		231		1,751
Total retail	169,943		46,886		275		217,104
Interchange	42,378		—		—		42,378
Other	969		908		—		1,877
Total revenue from contracts with customers	213,290		47,794		275		261,359
Alternative revenue and other	3,019		553		—		3,572
Total revenues	$	216,309	$	48,347	$	275	$	264,931