Consolidated Balance Sheets (unaudited) (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Property, plant and equipment, accumulated depreciation	$ 2,258,065	$ 2,250,950
Preferred stock, authorized shares	10,000,000us-gaap_PreferredStockSharesAuthorized	10,000,000us-gaap_PreferredStockSharesAuthorized
Preferred stock, issued shares	0us-gaap_PreferredStockSharesIssued	0us-gaap_PreferredStockSharesIssued
Common stock, authorized shares	200,000,000us-gaap_CommonStockSharesAuthorized	200,000,000us-gaap_CommonStockSharesAuthorized
Common stock, issued shares	107,417,644us-gaap_CommonStockSharesIssued	102,565,266us-gaap_CommonStockSharesIssued
Common stock, outstanding shares	107,417,644us-gaap_CommonStockSharesOutstanding	102,565,266us-gaap_CommonStockSharesOutstanding

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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The maximum number of common shares permitted to be issued by an entity's charter and bylaws.

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