Income Taxes (Details) (USD $)	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Mar. 31, 2010	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2010	Dec. 31, 2008
Income Taxes.
Consolidated provision for income taxes	$ 5,600,000	$ 4,500,000		$ 12,800,000	$ 14,400,000	$ 28,900,000		$ 24,600,000
Consolidated effective federal and state income tax rates (as a percent)	37.80%	37.70%		37.80%	47.50%	35.50%		38.90%
One-time non-cash charge			2,100,000
Amortization period of regulatory asset related to the tax benefits of cost of removal (in years)				eighteen year
Tax benefits of cost of removal flowed through to customers									11,100,000
Write-off of estimated portion of regulatory asset related to amortization period where rate recovery would no longer be probable			1,200,000
Amount received from Southwest Power Administration (SWPA) as reimbursement towards estimated future lifetime replacement cost of the electrical energy and capacity lost due							26,600,000
Increase in current tax liability on recognition of taxable income							10,000,000
Reserve balance related to unrecognized tax benefits							$ 359,000