Consolidated Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Proceeds from Income Tax Refunds	$ 35,573	$ 123	$ 11,584
Operating Activities:
Net income	192,277	112,294	78,800
Non-cash items included in net income:
Depreciation and amortization	138,337	133,189	126,402
Provision for deferred income taxes	144,269	23,532	21,449
Power and natural gas cost amortizations (deferrals), net	(14,821)	(9,408)	6,702
Amortization of debt expense	3,692	3,813	3,803
Amortization of investment in exchange power	2,450	2,450	2,450
Stock-based compensation expense	8,114	6,218	5,792
Equity-related AFUDC	(8,808)	(6,066)	(4,055)
Pension and other postretirement benefit expense	22,943	42,067	39,838
Amortization of Spokane Energy contract	12,417	11,414	10,492
Write-off of Reardan wind generation capitalized costs	0	2,534	0
Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax	(160,612)	0	0
Other	9,009	12,982	5,256
Pension Contributions	(32,000)	(44,263)	(44,000)
Changes in certain current assets and liabilities:
Accounts and notes receivable	16,425	(32,675)	8,100
Materials and supplies, fuel stock and natural gas stored	(19,394)	2,509	4,551
Decrease (Increase) Cash Collateral posted to Counterparties	(23,301)	(16,073)	9,695
Increase (Decrease) in Income Taxes Receivable	(36,110)	(5,006)	12,601
Other current assets	(7,117)	2,608	4,962
Accounts payable	(12,562)	(8,389)	30,189
Other current liabilities	32,060	8,827	(6,474)
Net cash provided by operating activities	267,268	242,557	316,553
Investing Activities:
Utility property capital expenditures (excluding equity-related AFUDC)	(325,516)	(294,363)	(271,187)
Other capital expenditures	(6,427)	(8,750)	(4,787)
Federal grant payments received	2,530	3,409	8,277
Cash Acquired in Excess of Payments to Acquire Business	15,007	0	0
Cash paid by subsidiaries for acquisitions, net of cash received	0	0	(50,310)
Decrease (increase) in funds held for clients	(18,931)	1,815	(6,811)
Purchase of securities available for sale	(12,267)	(35,949)	(100,374)
Sale and maturity of securities available for sale	14,612	22,960	137,999
Proceeds from Divestiture of Businesses, Net of Cash Divested	229,903	0	0
Other	(2,647)	(1,339)	(7,475)
Net cash used in investing activities	(103,736)	(312,217)	(294,668)
Financing Activities:
Net increase (decrease) in short-term borrowings	(66,000)	119,000	(9,000)
Borrowings from Ecova line of credit	0	3,000	33,000
Repayment of borrowings from Ecova line of credit	(46,000)	(11,000)	(14,000)
Proceeds from issuance of long-term debt	150,000	90,000	80,000
Redemption and maturity of long-term debt	(39,971)	(50,462)	(11,492)
Maturity of nonrecourse long-term debt of Spokane Energy	(16,407)	(14,965)	(13,669)
Cash received (paid) for settlement of interest rate swap agreements	5,429	2,901	(18,547)
Issuance of common stock, net of issuance costs	4,060	4,609	29,079
Payments for Repurchase of Common Stock	(79,856)	0	0
Cash dividends paid	(78,314)	(73,276)	(68,552)
Increase (decrease) in client fund obligations	16,216	11,278	(30,996)
Payments to Noncontrolling Interests	(54,179)	0	0
Payments for Repurchase of Redeemable Noncontrolling Interest	(20,871)	0	0
Other	1,930	(4,315)	3,094
Net cash provided by (used in) financing activities	(223,963)	76,770	(21,083)
Net increase (decrease) in cash and cash equivalents	(60,431)	7,110	802
Cash and cash equivalents at beginning of year	82,574	75,464	74,662
Cash and cash equivalents at end of year	22,143	82,574	75,464
Cash paid during the year:
Interest	73,526	75,411	74,900
Income taxes (net of refunds of $35,573, $123 and $11,584, respectively)	45,416	44,772	8,069
Non-cash financing and investing activities:
Accounts payable for capital expenditures	26,959	12,723	21,331
Valuation adjustment for redeemable noncontrolling interests	(15,873)	10,704	(10,104)
Escrow receivable included in investing activities	13,079	0	0
Stock Issued During Period, Value, Acquisitions	150,119	0	0
Contingent consideration by subsidiary for acquisition	$ 0	$ 0	$ 375