Segments and Related Information (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Intercompany Revenue										$ 170,000	$ 163,000	$ 145,000
Operating revenues		$ 237,066	$ 246,144	$ 219,173	$ 254,850	$ 229,607	$ 236,161	$ 216,813	$ 273,960	957,233	956,541	1,000,462
Depreciation and amortization										98,294	91,245	79,654
Total interest charges and financing costs										32,629	29,221	25,913
Income tax expense (benefit)										42,902	44,238	42,403
Net income (loss)		$ 14,658	$ 24,221	$ 12,625	$ 17,631	$ 13,625	$ 26,232	$ 12,512	$ 22,267	69,135	74,636	70,642
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues										850,343	835,417	830,245
Depreciation and amortization										81,299	77,036	65,978
Total interest charges and financing costs										29,749	26,494	23,448
Income tax expense (benefit)										40,547	40,654	39,621
Net income (loss)										65,002	69,398	59,060
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues										106,644	120,645	174,514
Depreciation and amortization										16,794	14,034	13,501
Total interest charges and financing costs										2,855	2,637	2,358
Income tax expense (benefit)										2,445	2,501	5,993
Net income (loss)										4,503	4,862	8,714
All Other
Segment Reporting Information [Line Items]
Operating revenues										1,130	1,396	1,085
Depreciation and amortization										201	175	175
Total interest charges and financing costs										25	90	107
Income tax expense (benefit)										(90)	1,083	(3,211)
Net income (loss)										(370)	376	2,868
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	[1]									957,233	956,541	1,000,462
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	[1]									849,946	834,998	829,748
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	[1]									106,157	120,147	169,629
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	[1]									1,130	1,396	1,085
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues										(884)	(917)	(5,382)
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										397	419	497
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues										487	498	4,885
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues										$ 0	$ 0	$ 0

[1] Operating revenues include $170 million, $163 million and $145 million of intercompany revenue for the years ended Dec. 31, 2016, 2015 and 2014 respectively. See Note 15 for further discussion of related party transactions by operating segment.