Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Current assets: [Abstract]
Cash and cash equivalents	$ 3,521	$ 4,211
Accounts receivable	66,700	68,228
Accrued unbilled revenue	64,946	57,987
Allowance for uncollectible accounts	(1,290)	(870)
Regulatory Assets, Current	42,362	69,178
Derivative instruments	17,031	2,719
Inventories	54,129	70,868
Deferred Gas Cost	15,926	17,094
Income taxes receivable	0	7,900
Other current assets	24,728	33,460
Total current assets	288,053	330,775
Non-current assets: [Abstract]
Property, plant, and equipment	3,208,816	3,089,380
Less: Accumulated depreciation	947,916	906,717
Total property, plant, and equipment, net	2,260,900	2,182,663
Inventory, Gas in Storage Underground, Noncurrent	100,184	114,552
Regulatory assets	357,530	370,711
Derivative instruments	3,265	27
Other investments	68,376	68,066
Other non-current assets	1,493	2,616
Total non-current assets	2,791,748	2,738,635
Total assets	3,079,801	3,069,410
Current liabilities: [Abstract]
Short-term Debt	53,300	270,035
Current maturities of long-term debt	39,989	24,973
Accounts payable	85,664	73,219
Taxes accrued	12,149	10,420
Interest accrued	5,966	5,873
Regulatory liabilities	40,290	29,927
Derivative instruments	1,315	22,092
Other current liabilities	35,844	41,148
Total current liabilities	274,517	477,687
Long-term debt	679,334	569,445
Deferred credits and other non-current liabilities: [Abstract]
Deferred tax liabilities	557,085	530,021
Regulatory liabilities	349,319	339,287
Pension and other postretirement benefit liabilities	225,725	223,105
Derivative instruments	913	3,447
Other non-current liabilities	142,411	145,446
Total deferred credits and other non-current liabilities	1,275,453	1,241,306
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	445,187	383,144
Retained earnings	412,261	404,990
Accumulated other comprehensive loss	(6,951)	(7,162)
Total equity	850,497	780,972
Total liabilities and equity	$ 3,079,801	$ 3,069,410

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.