Consolidated Balance Sheets - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Document Period End Date	Jun. 30, 2015
Current assets [Abstract]
Cash and cash equivalents	$ 4,466	$ 9,534	$ 17,240
Accounts receivable	32,041	69,818	38,621
Accrued unbilled revenue	12,760	57,963	14,592
Allowance for uncollectible accounts	(723)	(969)	(1,404)
Regulatory assets	63,016	68,562	38,265
Derivative instruments	1,023	243	11,191
Inventories	76,511	77,832	60,808
Gas reserves	18,214	20,020	20,373
Income taxes receivable	0	1,000	0
Deferred tax assets	12,693	23,785	4,915
Other current assets	15,348	34,772	14,518
Total current assets	235,349	362,560	219,119
Non-current assets [Abstract]
Property, plant, and equipment	3,042,671	2,992,560	2,965,226
Less: Accumulated depreciation	893,722	870,967	879,296
Total property, plant, and equipment, net	2,148,949	2,121,593	2,085,930
Gas reserves	121,355	129,280	130,280
Regulatory assets	342,806	368,908	267,248
Derivative instruments	1,369	0	1,202
Other investments	68,147	68,238	67,689
Restricted cash	4,500	3,000	3,000
Other non-current assets	9,404	11,366	12,646
Total non-current assets	2,696,530	2,702,385	2,567,995
Total assets	2,931,879	3,064,945	2,787,114
Current liabilities [Abstract]
Short-term debt	190,300	234,700	74,200
Current maturities of long-term debt	0	40,000	100,000
Accounts payable	49,505	91,366	68,973
Taxes accrued	8,782	10,031	15,769
Interest accrued	5,922	6,079	7,053
Regulatory liabilities	26,712	19,105	26,742
Derivative instruments	15,017	29,894	1,490
Other current liabilities	31,332	38,235	34,507
Total current liabilities	327,570	469,410	328,734
Long-term debt	621,700	621,700	621,700
Deferred credits and other non-current liabilities [Abstract]
Deferred tax liabilities	524,099	530,965	489,892
Regulatory liabilities	328,646	317,205	309,327
Pension and other postretirement benefit liabilities	233,554	236,735	145,861
Derivative instruments	1,077	3,515	191
Other non-current liabilities	118,269	118,094	120,423
Total deferred credits and other non-current liabilities	1,205,645	1,206,514	1,065,694
Commitments and contingencies (see Note 13)	0	0	0
Equity [Abstract]
Common stock - no par value; authorized 100,000 shares; issued and outstanding 27,363, 27,147, and 27,284 at June 30, 2015 and 2014 and December 31, 2014, respectively	378,887	375,117	369,315
Retained earnings	407,490	402,280	407,698
Accumulated other comprehensive loss	(9,413)	(10,076)	(6,027)
Total equity	776,964	767,321	770,986
Total liabilities and equity	$ 2,931,879	$ 3,064,945	$ 2,787,114