Segments and Related Information (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Intercompany Revenue										$ 177,000	$ 170,000	$ 163,000
Operating revenues		$ 262,983	$ 247,511	$ 230,026	$ 264,931	$ 237,066	$ 246,144	$ 219,173	$ 254,850	1,005,451	957,233	956,541
Depreciation and amortization										111,216	98,294	91,245
Total interest charges and financing costs										32,180	32,629	29,221
Income tax expense (benefit)										44,172	42,902	44,238
Net income (loss)		$ 20,431	$ 22,325	$ 14,241	$ 22,419	$ 14,658	$ 24,221	$ 12,625	$ 17,631	79,416	69,135	74,636
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues										882,388	850,343	835,417
Depreciation and amortization										88,946	81,299	77,036
Total interest charges and financing costs										29,396	29,749	26,494
Income tax expense (benefit)										38,866	40,547	40,654
Net income (loss)										70,876	65,002	69,398
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues										122,640	106,644	120,645
Depreciation and amortization										22,070	16,794	14,034
Total interest charges and financing costs										2,761	2,855	2,637
Income tax expense (benefit)										4,040	2,445	2,501
Net income (loss)										7,832	4,503	4,862
All Other
Segment Reporting Information [Line Items]
Operating revenues										1,207	1,130	1,396
Depreciation and amortization										200	201	175
Total interest charges and financing costs										23	25	90
Income tax expense (benefit)										1,266	(90)	1,083
Net income (loss)										708	(370)	376
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	[1]									1,005,451	957,233	956,541
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	[1]									881,891	849,946	834,998
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	[1]									122,353	106,157	120,147
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	[1]									1,207	1,130	1,396
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues										(784)	(884)	(917)
Depreciation and amortization										0	0	0
Total interest charges and financing costs										0	0	0
Income tax expense (benefit)										0	0	0
Net income (loss)										0	0	0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues										497	397	419
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues										287	487	498
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues										$ 0	$ 0	$ 0

[1] Operating revenues include $177 million, $170 million and $163 million of intercompany revenue for the years ended Dec. 31, 2017, 2016 and 2015 respectively. See Note 15 for further discussion of related party transactions by operating segment.