Provision (Benefit) for Income Taxes (Details 3) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Deferred tax assets:
Federal net operating losses ("NOLs")	$ 117,713	$ 93,445
NOLs for excess stock-based compensation	(9,615)	(9,676)
State NOL carryovers	14,626	13,686
Alternative minimum tax credits	2,092	2,092
Deferred Tax Assets, Other Loss Carryforwards	1,295	1,378
Deferred Tax Assets, Asset Retirement Obligations	28,628	31,413
Unrealized share-based compensation	9,957	9,096
Valuation allowance	(6,703)	(6,318)
Other	6,042	5,904
Total deferred tax assets	164,035	141,020
Deferred tax liabilities:
Deferred Tax Liabilities, Property, Plant and Equipment	(393,606)	(363,952)
Other	(919)	(3,299)
Deferred Tax Liabilities, Net	(394,525)	(367,251)
Net deferred tax liabilities	(230,490)	(226,231)
Net current deferred tax assets	10,715	8,262
Net non-current deferred tax liabilities	$ (241,205)	$ (234,493)