Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Current assets [Abstract]
Cash and cash equivalents	$ 5,218	$ 9,534	$ 17,929
Accounts receivable	68,531	69,818	87,264
Accrued unbilled revenue	30,076	57,963	33,515
Allowance for uncollectible accounts	(1,363)	(969)	(2,235)
Regulatory assets	67,702	68,562	27,834
Derivative instruments	658	243	15,846
Inventories	69,289	77,832	33,469
Gas reserves	19,112	20,020	21,990
Income taxes receivable	2,000	1,000	0
Deferred tax assets	13,491	23,785	4,915
Other current assets	17,271	34,772	13,595
Total current assets	291,985	362,560	254,122
Non-current assets [Abstract]
Property, plant, and equipment	3,017,754	2,992,560	2,939,760
Less: Accumulated depreciation	883,254	870,967	868,257
Total property, plant, and equipment, net	2,134,500	2,121,593	2,071,503
Gas reserves	125,187	129,280	134,894
Regulatory assets	348,421	368,908	285,046
Derivative instruments	117	0	1,078
Other investments	68,614	68,238	67,288
Restricted cash	3,000	3,000	4,000
Other non-current assets	10,577	11,366	12,453
Total non-current assets	2,690,416	2,702,385	2,576,262
Total assets	2,982,401	3,064,945	2,830,384
Current liabilities [Abstract]
Short-term debt	156,200	234,700	32,600
Current maturities of long-term debt	40,000	40,000	80,000
Accounts payable	62,904	91,366	89,201
Taxes accrued	17,755	10,031	34,146
Interest accrued	10,427	6,079	11,144
Regulatory liabilities	24,263	19,105	37,686
Derivative instruments	23,242	29,894	1,191
Other current liabilities	35,950	38,235	38,069
Total current liabilities	370,741	469,410	324,037
Long-term debt	621,700	621,700	661,700
Deferred credits and other non-current liabilities [Abstract]
Deferred tax liabilities	523,929	530,965	489,108
Regulatory liabilities	326,424	317,205	308,858
Pension and other postretirement benefit liabilities	235,516	236,735	147,733
Derivative instruments	1,117	3,515	96
Other non-current liabilities	118,059	118,094	119,376
Total deferred credits and other non-current liabilities	1,205,045	1,206,514	1,065,171
Commitments and contingencies (see Note 13)	0	0	0
Equity [Abstract]
Common stock - no par value; authorized 100,000 shares; issued and outstanding 27,332, 27,132, and 27,284 at March 31, 2015 and 2014 and December 31, 2014, respectively	376,656	375,117	366,560
Retained earnings	418,003	402,280	419,109
Accumulated other comprehensive loss	(9,744)	(10,076)	(6,193)
Total equity	784,915	767,321	779,476
Total liabilities and equity	$ 2,982,401	$ 3,064,945	$ 2,830,384