Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets: [Abstract]
Cash and cash equivalents	$ 3,472	$ 3,521
Accounts receivable	68,362	66,700
Accrued unbilled revenue	62,381	64,946
Allowance for uncollectible accounts	(956)	(1,290)
Regulatory Assets, Current	45,781	42,362
Derivative instruments	1,735	17,031
Inventories	47,973	54,129
Deferred Gas Cost	15,704	15,926
Other current assets	25,484	24,728
Total current assets	269,936	288,053
Non-current assets: [Abstract]
Property, Plant and Equipment, Gross	3,215,451	3,208,816
Less: Accumulated depreciation	960,477	947,916
Total property, plant, and equipment, net	2,254,974	2,260,900
Inventory, Gas in Storage Underground, Noncurrent	84,053	100,184
Regulatory assets	356,608	357,530
Derivative instruments	1,306	3,265
Other investments	66,363	68,376
Other non-current assets	6,506	1,493
Total non-current assets	2,769,810	2,791,748
Total assets	3,039,746	3,079,801
Current liabilities: [Abstract]
Short-term Debt	54,200	53,300
Current maturities of long-term debt	96,703	39,989
Accounts payable	112,308	85,664
Taxes accrued	18,883	12,149
Interest accrued	6,773	5,966
Regulatory Liability, Current	34,013	40,290
Derivative instruments	18,722	1,315
Other current liabilities	40,248	35,844
Total current liabilities	381,850	274,517
Long-term debt	683,184	679,334
Deferred credits and other non-current liabilities: [Abstract]
Deferred tax liabilities	270,526	557,085
Regulatory Liability, Noncurrent	586,093	349,319
Pension and other postretirement benefit liabilities	223,333	225,725
Derivative instruments	4,649	913
Other Liabilities, Noncurrent	147,335	142,411
Total deferred credits and other non-current liabilities	1,231,936	1,275,453
Commitments and contingencies (see Note 14 and Note 15)	0	0
Equity: [Abstract]
Common stock - no par value; authorized 100,000 shares; issued and outstanding 28,630 and 27,427 at December 31, 2016 and 2015, respectively	448,865	445,187
Retained earnings	302,349	412,261
Accumulated other comprehensive loss	(8,438)	(6,951)
Total equity	742,776	850,497
Total liabilities and equity	$ 3,039,746	$ 3,079,801

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.