Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 3,281	$ 3,554
Accumulated depreciation on property, plant and equipment	1,757,456	1,730,750
Accumulated amortization on other assets	$ 5,999	$ 5,761

Amount of accumulated amortization of debt issuance costs.

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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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A valuation allowance for trade and other receivables due to an Entity within one year (or the normal operating cycle, whichever is longer) that are expected to be uncollectible.

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