Accounting For Income Taxes (Schedule Of Effective Income Tax Rate Reconciliation) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Accounting For Income Taxes [Abstract]
Federal income taxes at statutory rates	$ 56,060	$ 51,137	$ 47,182
Tax effect of regulatory treatment of utility plant differences	1,798	2,761	1,858
State income tax expense	687	624	2,746
Settlement of prior year tax returns and adjustment of tax reserves	163	(1,030)	(2,726)
Manufacturing deduction	(1,099)	(1,630)	(1,091)
Kettle Falls tax credit			(1,622)
Other	(977)	(705)	(24)
Total income tax expense	$ 56,632	$ 51,157	$ 46,323
Effective income tax rate reconciliation, at federal statutory income tax rate	35.00%	35.00%	35.00%