Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Proceeds from Income Tax Refunds	$ 18,861	$ 37,200	$ 35,573
Operating Activities:
Net income	137,316	123,317	192,277
Non-cash items included in net income:
Depreciation and amortization	164,925	147,835	138,337
Deferred Income Tax Expense (Benefit) Continuing Operations and Discontinued Operations	124,543	51,801	144,269
Power and natural gas cost amortizations (deferrals), net	16,835	21,358	(14,821)
Amortization of debt expense	3,477	3,526	3,692
Amortization of Power Contracts Emission Credits	2,450	2,450	2,450
Stock-based compensation expense	7,891	6,914	8,114
Equity-related AFUDC	(8,475)	(8,331)	(8,808)
Pension and other postretirement benefit expense	38,786	37,050	22,943
Amortization of Spokane Energy contract	14,694	13,508	12,417
Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax	0	(777)	(160,612)
Increase (Decrease) in Regulatory Assets and Liabilities	(26,245)	4,569	7,906
Increase (Decrease) In Regulatory Assets And Liabilities Associated with Alternative Revenue Programs	(29,789)	(10,933)	0
Other	5,557	(517)	1,103
Pension Contributions	(12,000)	(12,000)	(32,000)
Payments For (Proceeds From) Derivative Instrument Operating Activities	(53,966)	0	0
Changes in certain current assets and liabilities:
Accounts and notes receivable	(17,170)	(10,538)	16,425
Materials and supplies, fuel stock and stored natural gas	834	12,208	(19,394)
Increase (Decrease) in Deposit Assets	10,712	(13,301)	(23,301)
Increase (Decrease) in Income Taxes Receivable	(33,923)	19,772	(36,110)
Other current assets	(3,907)	2,338	(7,117)
Accounts payable	5,176	(8,138)	(12,562)
Other current liabilities	10,546	(6,471)	32,060
Net cash provided by operating activities	358,267	375,640	267,268
Investing Activities:
Utility property capital expenditures (excluding equity-related AFUDC)	(406,644)	(393,425)	(325,516)
Other capital expenditures	(353)	(885)	(6,427)
Cash Acquired in Excess of Payments to Acquire Business			15,007
Issuance of Notes Receivable	(10,094)	(2,307)	(1,200)
Repayments from Notes Receivable	5,000	0	0
Investments made by subsidiaries	(13,097)	(1,944)	(1,072)
Cash received (paid) in acquisition, net	0	(95)
Increase in funds held for clients	0	0	(18,931)
Purchase of securities available for sale	0	0	(12,267)
Sale and maturity of securities available for sale	0	0	14,612
Proceeds from Divestiture of Businesses, Net of Cash Divested	0	13,856	229,903
Other	(7,278)	(3,027)	2,155
Net cash used in investing activities	(432,466)	(387,827)	(103,736)
Financing Activities:
Net increase (decrease) in borrowings from committed line of credit	15,000	0	(66,000)
Repayment of borrowings from Ecova line of credit	0	0	(46,000)
Proceeds from issuance of long-term debt	245,000	100,000	150,000
Redemption and maturity of long-term debt and capital leases	(163,167)	(2,905)	(39,971)
Maturity of nonrecourse long-term debt of Spokane Energy	0	(1,431)	(16,407)
Issuance of common stock, net of issuance costs	66,953	1,560	4,060
Payments for Repurchase of Common Stock	0	(2,920)	(79,856)
Cash dividends paid	(87,154)	(82,397)	(78,314)
Increase in client fund obligations	0	0	16,216
Payments to Noncontrolling Interests	0	0	(54,179)
Payments for Repurchase of Redeemable Noncontrolling Interest	0	0	(20,871)
Other	(4,410)	(11,379)	7,359
Net cash provided by (used in) financing activities	72,222	528	(223,963)
Net decrease in cash and cash equivalents	(1,977)	(11,659)	(60,431)
Cash and cash equivalents at beginning of year	10,484	22,143	82,574
Cash and cash equivalents at end of year	8,507	10,484	22,143
Cash paid (received) during the year:
Interest	86,319	79,673	73,526
Income Taxes Paid, Net	(13,458)	(9,961)	45,416
Non-cash financing and investing activities:
Accounts payable for capital expenditures	30,252	35,248	26,959
Valuation adjustment for redeemable noncontrolling interests	0	0	(15,873)
Escrow receivable included in investing activities	0	0	13,079
Stock Issued During Period, Value, Acquisitions Net of Issuance Costs	$ 0	$ 0	$ 150,119