Income Taxes (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Deferred Tax Liabilities:
Accelerated Depreciation	$ 248,515	$ 238,912
Prepaid Costs Currently Deductible	4,597	4,178
Other	2,451	2,414
Total Deferred Tax Liabilities	255,563	245,504
Deferred Tax Assets:
Accrued Costs Not Yet Deductible	6,138	6,017
Allowance for Doubtful Accounts	1,163	582
Net Operating Loss Carry Forwards and Credits	12,103	25,495
Deferred Revenues	1,300	243
Stock Based Compensation	8,295	7,801
Total Deferred Tax Assets	28,999	40,138
Deferred Income Taxes, net	$ 226,564	$ 205,366