Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Current assets: [Abstract]
Cash and cash equivalents	$ 5,463	$ 4,211	$ 4,466
Accounts receivable	23,353	68,228	32,041
Accrued unbilled revenue	14,175	57,987	12,760
Allowance for uncollectible accounts	(570)	(870)	(723)
Regulatory Assets, Current	49,004	69,178	63,016
Derivative instruments	7,445	2,719	1,023
Inventories	66,171	70,868	76,511
Deferred Gas Cost	15,707	17,094	18,214
Income taxes receivable	0	7,900	0
Deferred tax assets	0	0	12,693
Other current assets	21,312	33,460	14,007
Total current assets	202,060	330,775	234,008
Non-current assets: [Abstract]
Property, plant, and equipment	3,146,631	3,089,380	3,042,671
Less: Accumulated depreciation	932,179	906,717	893,722
Total property, plant, and equipment, net	2,214,452	2,182,663	2,148,949
Inventory, Gas in Storage Underground, Noncurrent	108,286	114,552	121,355
Regulatory assets	344,969	370,711	342,806
Derivative instruments	3,541	27	1,369
Other investments	67,868	68,066	68,147
Restricted cash	0	0	4,500
Other non-current assets	1,968	2,616	2,782
Total non-current assets	2,741,084	2,738,635	2,689,908
Total assets	2,943,144	3,069,410	2,923,916
Current liabilities: [Abstract]
Short-term Debt	152,800	270,035	190,300
Current maturities of long-term debt	24,987	24,973	0
Accounts payable	57,756	73,219	49,505
Taxes accrued	6,237	10,420	8,782
Interest accrued	5,793	5,873	5,922
Regulatory liabilities	27,300	29,927	26,712
Derivative instruments	3,471	22,092	15,017
Other current liabilities	35,289	41,148	31,332
Total current liabilities	313,633	477,687	327,570
Long-term debt	570,045	569,445	613,737
Deferred credits and other non-current liabilities: [Abstract]
Deferred tax liabilities	554,400	530,021	524,099
Regulatory liabilities	341,259	339,287	328,646
Pension and other postretirement benefit liabilities	219,049	223,105	233,554
Derivative instruments	474	3,447	1,077
Other non-current liabilities	144,285	145,446	118,269
Total deferred credits and other non-current liabilities	1,259,467	1,241,306	1,205,645
Commitments and contingencies (see Note 13)	0	0	0
Equity: [Abstract]
Common stock - no par value; authorized 100,000 shares; issued and outstanding 27,550, 27,363, and 27,427 at June 30, 2016 and 2015, and December 31, 2015, respectively	388,967	383,144	378,887
Retained earnings	417,857	404,990	407,490
Accumulated other comprehensive loss	(6,825)	(7,162)	(9,413)
Total equity	799,999	780,972	776,964
Total liabilities and equity	$ 2,943,144	$ 3,069,410	$ 2,923,916

Carrying value as of the balance sheet date of liabilities incurred (and for which invoices have typically been received) and payable to vendors for goods and services received that are used in an entity's business. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

Name:	us-gaap_AccountsPayableCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amounts due from customers or clients, within one year of the balance sheet date (or the normal operating cycle, whichever is longer) for goods or services (including trade receivables) that have been delivered or sold in the normal course of business.