Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Activities:
Net income	$ 136,598	$ 115,932	$ 137,316
Non-cash items included in net income:
Depreciation and amortization	187,318	175,655	164,925
Deferred income tax expense	8,570	69,657	124,543
Power and natural gas cost deferrals, net	10,263	11,741	16,835
Amortization of debt expense	2,967	3,254	3,477
Amortization of investment in exchange power	2,450	2,450	2,450
Stock-based compensation expense	5,367	7,359	7,891
Equity-related AFUDC	(6,554)	(6,669)	(8,475)
Pension and other postretirement benefit expense	32,017	37,074	38,786
Amortization of Spokane Energy contract	0	0	14,694
Other regulatory assets and liabilities and deferred debits and credits	27,512	(9,144)	(26,245)
Change in decoupling regulatory deferral	(1,288)	24,179	(29,789)
Other	1,114	1,860	5,557
Contributions to defined benefit pension plan	(22,000)	(22,000)	(12,000)
Cash paid on settlement of interest rate swap derivatives	(32,174)	(11,302)	(53,966)
Cash received on settlement of interest rate swap derivatives	5,594	2,479	0
Changes in certain current assets and liabilities:
Accounts and notes receivable	15,474	(9,270)	(17,170)
Materials and supplies, fuel stock and stored natural gas	(5,807)	(4,767)	834
Collateral posted for derivative instruments	(4,128)	(22,394)	10,712
Income taxes receivable	2,021	53,414	(33,923)
Other current assets	(2,589)	(2,106)	(3,907)
Accounts payable	(470)	(8,162)	5,176
Other current liabilities	(370)	1,058	10,546
Net cash provided by operating activities	361,885	410,298	358,267
Investing Activities:
Utility property capital expenditures (excluding equity-related AFUDC)	(424,350)	(412,339)	(406,644)
Issuance of notes receivable at subsidiaries	(3,555)	(3,700)	(10,094)
Repayments from notes receivable at subsidiaries	871	0	5,000
Equity and property investments made by subsidiaries	(13,283)	(13,680)	(13,097)
Distributions received from investments	2,228	1,915	0
Other	(2,343)	(6,299)	(7,631)
Net cash used in investing activities	(440,432)	(434,103)	(432,466)
Financing Activities:
Net increase (decrease) in short-term borrowings	84,603	(15,000)	15,000
Proceeds from issuance of long-term debt	374,621	90,000	245,000
Maturity of long-term debt and capital leases	(277,438)	(3,287)	(163,167)
Issuance of common stock, net of issuance costs	1,207	56,380	66,953
Cash dividends paid	(98,046)	(92,460)	(87,154)
Other	(7,916)	(4,163)	(4,410)
Net cash provided by financing activities	77,031	31,470	72,222
Net increase (decrease) in cash and cash equivalents	(1,516)	7,665	(1,977)
Cash and cash equivalents at beginning of year	16,172	8,507	10,484
Cash and cash equivalents at end of year	14,656	16,172	8,507
Cash paid (received) during the year:
Interest	97,437	95,499	86,319
Income taxes paid	17,801	5,579	5,403
Income tax refunds	(3,025)	(47,086)	(18,861)
Non-cash financing and investing activities:
Accounts payable for capital expenditures	$ 31,868	$ 31,157	$ 30,252