Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Proceeds from Income Tax Refunds	$ 37,200	$ 35,573	$ 123
Operating Activities:
Net income	123,317	192,277	112,294
Non-cash items included in net income:
Depreciation and amortization	147,835	138,337	133,189
Provision for deferred income taxes	51,801	144,269	23,532
Power and natural gas cost amortizations (deferrals), net	21,358	(14,821)	(9,408)
Amortization of debt expense	3,526	3,692	3,813
Amortization of Power Contracts Emission Credits	2,450	2,450	2,450
Stock-based compensation expense	6,914	8,114	6,218
Equity-related AFUDC	(8,331)	(8,808)	(6,066)
Pension and other postretirement benefit expense	37,050	22,943	42,067
Amortization of Spokane Energy contract	13,508	12,417	11,414
Write-off of wind generation capitalized costs	0	0	2,534
Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax	(777)	(160,612)	0
Other	(6,881)	9,009	12,982
Pension Contributions	(12,000)	(32,000)	(44,263)
Changes in certain current assets and liabilities:
Accounts and notes receivable	10,538	(16,425)	32,675
Materials and supplies, fuel stock and stored natural gas	(12,208)	19,394	(2,509)
Increase (Decrease) in Deposit Assets	(13,301)	(23,301)	(16,073)
Increase (Decrease) in Income Taxes Receivable	19,772	(36,110)	(5,006)
Other current assets	(2,338)	7,117	(2,608)
Accounts payable	(8,138)	(12,562)	(8,389)
Other current liabilities	(6,471)	32,060	8,827
Net cash provided by operating activities	375,640	267,268	242,557
Investing Activities:
Utility property capital expenditures (excluding equity-related AFUDC)	(393,425)	(325,516)	(294,363)
Other capital expenditures	(885)	(6,427)	(8,750)
Federal and state grant payments received	2,730	2,530	3,409
Cash Acquired in Excess of Payments to Acquire Business		15,007	0
Cash received (paid) in acquisition, net	(95)
Decrease (increase) in funds held for clients	0	(18,931)	1,815
Purchase of securities available for sale	0	(12,267)	(35,949)
Sale and maturity of securities available for sale	0	14,612	22,960
Proceeds from Divestiture of Businesses, Net of Cash Divested	13,856	229,903	0
Other	(10,008)	(2,647)	(1,339)
Net cash used in investing activities	(387,827)	(103,736)	(312,217)
Financing Activities:
Net increase (decrease) in short-term borrowings	0	(66,000)	119,000
Borrowings from Ecova line of credit	0	0	3,000
Repayment of borrowings from Ecova line of credit	0	(46,000)	(11,000)
Proceeds from issuance of long-term debt	100,000	150,000	90,000
Redemption and maturity of long-term debt and capital leases	(2,905)	(39,971)	(50,462)
Maturity of nonrecourse long-term debt of Spokane Energy	(1,431)	(16,407)	(14,965)
Cash received (paid) for settlement of interest rate swap agreements	(9,326)	5,429	2,901
Issuance of common stock, net of issuance costs	1,560	4,060	4,609
Payments for Repurchase of Common Stock	(2,920)	(79,856)	0
Cash dividends paid	(82,397)	(78,314)	(73,276)
Increase in client fund obligations	0	16,216	11,278
Payments to Noncontrolling Interests	0	(54,179)	0
Payments for Repurchase of Redeemable Noncontrolling Interest	0	(20,871)	0
Other	(2,053)	1,930	(4,315)
Net cash provided by (used in) financing activities	528	(223,963)	76,770
Net increase (decrease) in cash and cash equivalents	(11,659)	(60,431)	7,110
Cash and cash equivalents at beginning of year	22,143	82,574	75,464
Cash and cash equivalents at end of year	10,484	22,143	82,574
Cash paid (received) during the year:
Interest	79,673	73,526	75,411
Income Taxes Paid, Net	(9,961)	45,416	44,772
Non-cash financing and investing activities:
Accounts payable for capital expenditures	35,248	26,959	12,723
Valuation adjustment for redeemable noncontrolling interests	0	(15,873)	10,704
Escrow receivable included in investing activities	0	13,079	0
Stock Issued During Period, Value, Acquisitions Net of Issuance Costs	$ 0	$ 150,119	$ 0