Income tax expense (benefit) statutory rate versus actual rate (Details 5) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Computed tax at federal statutory rate
Computed tax at federal statutory rate	$ (15,914,000)	$ 117,679,000	$ 128,293,000
Federal statutory rate (in hundredths)	35.00%	35.00%	35.00%
Increases (reductions) resulting from:
State income taxes, net of federal income tax benefit (expense)	2,469,000	10,653,000	10,210,000
State income tax rate (in hundredths)	(5.40%)	3.20%	2.80%
Resolution of tax matters and uncertain tax positions	2,559,000	(3,906,000)	667,000
Resolution of tax matters and uncertain tax positions rate (in hundredths)	(5.60%)	(1.20%)	0.20%
Federal renewable energy credit	(3,401,000)	(3,485,000)	(2,185,000)
Federal renewable energy credit rate (in hundreths)	7.50%	(1.00%)	(0.60%)
Depletion allowance	(3,728,000)	(3,266,000)	(2,810,000)
Depletion allowance rate (in hundredths)	8.20%	(1.00%)	(0.80%)
Deductible K-Plan dividends	(2,829,000)	(2,282,000)	(2,309,000)
Deductible K-Plan dividends rate (in hundredths)	6.20%	(0.70%)	(0.60%)
Deferred tax rate changes	(3,083,000)	(417,000)	(1,262,000)
Deferred tax rate changes rate (in hundredths)	6.80%	(0.10%)	(0.30%)
AFUDC equity	(1,500,000)	(873,000)	(1,494,000)
AFUDC equity rate (in hundredths)	3.30%	(0.30%)	(0.40%)
Other	(5,719,000)	(3,829,000)	(6,580,000)
Other rate (in hundredths)	12.50%	(1.10%)	(1.90%)
Total income tax expense (benefit)	(31,146,000)	110,274,000	122,530,000
Total income tax expense (benefit) rate (in hundredths)	68.50%	32.80%	33.40%
Temporary differences [Abstract]
Undistributed earnings from foreign operations	7,700,000
Deferred tax liabilities, undistributed foreign earnings	$ 2,000,000