Segments and Related Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Revenues excluding intersegment									$ 861,842	$ 875,790	$ 827,291
Intersegment revenues									0	0	0
Total operating revenues	217,395	226,475	194,173	223,799	214,364	224,468	198,850	238,108	861,842	875,790	827,291
Depreciation and amortization									69,234	68,574	63,669
Interest charges and financing costs									22,937	23,993	23,478
Income tax expense (benefit)									29,558	33,614	26,112
Net income	7,131	22,200	5,742	14,878	7,912	19,973	8,478	14,643	49,951	51,006	42,749
Regulated Electric [Member]
Segment Reporting Information [Line Items]
Revenues excluding intersegment									757,565	755,136	708,179
Intersegment revenues									355	405	360
Total operating revenues									757,920	755,541	708,539
Depreciation and amortization									59,768	58,800	54,414
Interest charges and financing costs									20,303	21,181	20,738
Income tax expense (benefit)									27,164	32,656	24,819
Net income									45,377	47,093	37,773
Regulated Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenues excluding intersegment									103,100	119,447	118,076
Intersegment revenues									727	1,581	1,122
Total operating revenues									103,827	121,028	119,198
Depreciation and amortization									9,251	9,599	9,037
Interest charges and financing costs									2,554	2,675	2,597
Income tax expense (benefit)									2,113	1,995	1,836
Net income									3,094	2,964	3,325
All Other [Member]
Segment Reporting Information [Line Items]
Revenues excluding intersegment									1,177	1,207	1,036
Intersegment revenues									0	0	0
Total operating revenues									1,177	1,207	1,036
Depreciation and amortization									215	175	218
Interest charges and financing costs									80	137	143
Income tax expense (benefit)									281	(1,037)	(543)
Net income									1,480	949	1,651
Reconciling Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues excluding intersegment									0	0	0
Intersegment revenues									(1,082)	(1,986)	(1,482)
Total operating revenues									(1,082)	(1,986)	(1,482)
Depreciation and amortization									0	0	0
Interest charges and financing costs									0	0	0
Income tax expense (benefit)									0	0	0
Net income									$ 0	$ 0	$ 0