Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Current assets: [Abstract]
Cash and cash equivalents	$ 4,211	$ 9,534
Accounts receivable	68,228	69,818
Accrued unbilled revenue	57,987	57,963
Allowance for uncollectible accounts	(870)	(969)
Regulatory Assets, Current	69,178	68,562
Derivative instruments	2,719	243
Inventories	70,868	77,832
Deferred Gas Cost	17,094	20,020
Income taxes receivable	7,900	1,000
Deferred tax assets	0	23,785
Other current assets	34,748	34,772
Total current assets	332,063	362,560
Non-current assets: [Abstract]
Property, plant, and equipment	3,089,380	2,992,560
Less: Accumulated depreciation	906,717	870,967
Total property, plant, and equipment, net	2,182,663	2,121,593
Inventory, Gas in Storage Underground, Noncurrent	114,552	129,280
Regulatory assets	370,711	368,908
Derivative instruments	27	0
Other investments	68,066	68,238
Restricted cash	0	3,000
Other non-current assets	8,610	11,366
Total non-current assets	2,744,629	2,702,385
Total assets	3,076,692	3,064,945
Current liabilities: [Abstract]
Short-term Debt	270,035	234,700
Current maturities of long-term debt	25,000	40,000
Accounts payable	73,219	91,366
Taxes accrued	10,420	10,031
Interest accrued	5,873	6,079
Regulatory liabilities	29,927	19,105
Derivative instruments	22,092	29,894
Other current liabilities	41,148	38,235
Total current liabilities	477,714	469,410
Long-term debt	576,700	621,700
Deferred credits and other non-current liabilities: [Abstract]
Deferred tax liabilities	530,021	530,965
Regulatory liabilities	339,287	317,205
Pension and other postretirement benefit liabilities	223,105	236,735
Derivative instruments	3,447	3,515
Other non-current liabilities	145,446	118,094
Total deferred credits and other non-current liabilities	1,241,306	1,206,514
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 27,427 and 27,284 at December 31, 2015 and 2014, respectively	383,144	375,117
Retained earnings	404,990	402,280
Accumulated other comprehensive loss	(7,162)	(10,076)
Total equity	780,972	767,321
Total liabilities and equity	$ 3,076,692	$ 3,064,945

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.