Income Taxes Effective Tax Rate Reconciliation (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Effective tax rate reconciliation
Federal statutory rate	35.00%	35.00%	35.00%	35.00%
Flow-through repairs deductions	(17.70%)	14.60%	(16.80%)	(22.90%)
Flow-through of state bonus depreciaton deduction	(4.50%)	2.20%	(4.30%)	(5.20%)
Production tax credits	(2.80%)	0.00%	(2.80%)	0.00%
Prior year permanent return to accrual adjustments	0.00%	15.00%	0.60%	(4.50%)
Recognition of state net operating loss benefit / valuation allowance release	0.00%	0.40%	0.00%	(0.10%)
State income tax and other, net	0.40%	2.30%	0.00%	2.50%
Effective income tax rate	10.40%	69.50%	11.70%	4.80%
Significant differences from federal statutory rate
Income (Loss) Before Income Taxes	$ 17,462	$ (12,360)	$ 76,855	$ 41,698
Income tax calculated at 35% federal statutory rate	6,112	(4,326)	26,899	14,594
Permanent or flow through adjustments:
Flow-through repairs deductions	(3,086)	(1,808)	(12,898)	(9,547)
Flow-through of state bonus depreciation deduction	(786)	(276)	(3,287)	(2,159)
Production tax credits	(482)	0	(2,152)	0
Prior year permanent return to accrual adjustments	0	(1,857)	541	(1,857)
Recognition of state net operating loss benefit / valuation allowance release	0	(51)	0	(51)
State income tax and other, net	57	(270)	(138)	1,009
Total permanent or flow-through adjustments	(4,297)	(4,262)	(17,934)	(12,605)
Income tax expense (benefit)	$ 1,815	$ (8,588)	$ 8,965	$ 1,989
Internal Revenue Service (IRS) [Member]
Effective tax rate reconciliation
Federal statutory rate			35.00%