Income Taxes (Details) (USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Current
Federal	$ 24,618,000	$ 39,749,000	$ 29,879,000
State	563,000	(1,714,000)	3,424,000
Total	25,181,000	38,035,000	33,303,000
Deferred
Federal	57,704,000	46,599,000	37,981,000
State	23,225,000	28,145,000	23,828,000
Total	80,929,000	74,744,000	61,809,000
Total income tax expense	106,110,000	112,779,000	95,112,000
Reconciliation of actual provision for income taxes and provision for income taxes resulting from the use of federal statutory income tax
Statutory federal income tax rate (as a percent)	35.00%	35.00%	35.00%
Computed "expected" federal income tax	83,244,000	82,316,000	69,475,000
State income tax, net of federal income tax benefit	9,609,000	8,989,000	6,638,000
Deferred tax adjustment related to change in overall state tax rate	13,596,000	19,068,000	18,973,000
Other, net	(339,000)	2,406,000	26,000
Total income tax expense	106,110,000	112,779,000	95,112,000
Deferred Tax Liabilities
Property, plant and equipment	1,204,735,000	1,068,762,000
Hedging liabilities / receivables	19,915,000	68,670,000
Prepaid expenses and other	736,000	9,261,000
Total	1,225,386,000	1,146,693,000
Deferred Tax Assets
Alternative minimum tax credit	125,862,000	101,290,000
Net operating loss	137,422,000	113,496,000
Foreign tax credits	4,923,000	4,685,000
Pension and other post-retirement benefits	16,498,000	19,892,000
Items accrued for financial reporting purposes and other	52,806,000	49,606,000
Total	337,511,000	288,969,000
Net deferred tax liabilities	887,875,000	857,724,000
Reconciliation of the beginning and ending amount of unrecognized tax benefits
Unrecognized tax benefit balance at the beginning of the period			500,000
Reductions for tax positions of prior years			(500,000)
State tax effected net operating losses
Tax benefits related to employee stock-based compensation included in net operating loss carryforwards	66,900,000
State Tax Reporting
State tax effected net operating losses
Net operating loss carryforwards	378,200,000
Federal
State tax effected net operating losses
Net operating loss carryforwards	$ 398,600,000

The cumulative amount of the estimated future tax effects attributable to prepaid expenses and other temporary differences not otherwise specified in this taxonomy that were expensed for tax purposes but capitalized in conformity with generally accepted accounting principles, or which were recognized as revenue under GAAP but not for tax purposes, which will reverse in future periods.

No definition available.