Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands, shares in Millions	Dec. 31, 2019	Dec. 31, 2018
Accounts receivable, non-trade	$ 34,468	$ 8,183
Accumulated depreciation	$ 1,666,582	$ 1,634,394
Common Units
Limited partners, units issued	411.1	410.3
Limited partners, units outstanding	411.1	410.3
Preferred Units
Limited partners, units issued	15.8	15.8
Limited partners, units outstanding	15.8	15.8

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 number of limited partner units issued.

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The sum of amounts currently receivable other than from customers. For classified balance sheets, represents the current amount receivable, that is amounts expected to be collected within one year or the normal operating cycle, if longer.

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