Summary of Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Depreciation, Depletion, Accretion and Amortization
Depreciation	$ 328,072	$ 349,460	$ 361,530
Depletion	11,195	10,337	10,143
Accretion	8,195	8,641	8,802
Amortization of leaseholds and capitalized leases	225	195	180
Amortization of intangibles	14,032	13,460	13,957
Total	$ 361,719	$ 382,093	$ 394,612

Amount recognized for the passage of time, typically for liabilities, that have been discounted to their net present values. Excludes accretion associated with asset retirement obligations.

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The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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- Definition

The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

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