Property, Plant and Equipment (Details) (USD $)	3 Months Ended	12 Months Ended
Mar. 31, 2013 site	Mar. 31, 2012	Dec. 31, 2012
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 622,148,000		$ 611,685,000
Less accumulated amortization and depreciation	251,952,000		246,211,000
Property, plant and equipment, net	370,196,000		365,474,000
Number of sites	274
Accelerated depreciation expense	800,000	2,000,000	8,400,000
Plant in Service [Member]
Property, plant and equipment [Abstract]
Total property, plant and equipment	592,900,000		586,216,000
Plant under Construction [Member]
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 29,248,000		$ 25,469,000

The cumulative amount of depreciation, depletion and amortization (related to property, plant and equipment, but not including land) that has been recognized in the income statement.

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Gross amount of long-lived physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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- Definition

Amount, net of accumulated depreciation, depletion and amortization, of long-lived physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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