Segments and Related Information (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Intercompany Revenue										$ 163,000	$ 145,000	$ 137,000
Operating revenues		$ 229,607	$ 236,161	$ 216,813	$ 273,960	$ 256,160	$ 231,046	$ 228,114	$ 285,142	956,541	1,000,462	923,038
Depreciation and amortization										91,245	79,654	76,897
Total interest charges and financing costs										29,221	25,913	25,816
Income tax expense (benefit)										44,238	42,403	36,409
Net income (loss)		$ 13,625	$ 26,232	$ 12,512	$ 22,267	$ 14,355	$ 20,030	$ 12,022	$ 24,235	74,636	70,642	59,468
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues										835,417	830,245	789,518
Depreciation and amortization										77,036	65,978	64,237
Total interest charges and financing costs										26,494	23,448	22,966
Income tax expense (benefit)										40,654	39,621	33,691
Net income (loss)										69,398	59,060	51,334
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues										120,645	174,514	134,834
Depreciation and amortization										14,034	13,501	12,485
Total interest charges and financing costs										2,637	2,358	2,749
Income tax expense (benefit)										2,501	5,993	4,623
Net income (loss)										4,862	8,714	6,501
All Other
Segment Reporting Information [Line Items]
Operating revenues										1,396	1,085	1,003
Depreciation and amortization										175	175	175
Total interest charges and financing costs										90	107	101
Income tax expense (benefit)										1,083	(3,211)	(1,905)
Net income (loss)										376	2,868	1,633
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	[1]									956,541	1,000,462	923,038
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	[1]									834,998	829,748	789,168
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	[1]									120,147	169,629	132,867
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	[1]									1,396	1,085	1,003
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues										(917)	(5,382)	(2,317)
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										419	497	350
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues										498	4,885	1,967
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues										$ 0	$ 0	$ 0

[1] Operating revenues include $163 million, $145 million and $137 million of intercompany revenue for the years ended Dec. 31, 2015, 2014 and 2013 respectively. See Note 15 for further discussion of related party transactions by operating segment.