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Consolidated Condensed Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Assets: |  |
Allowance for doubtful accounts, current | $ 15,288 | $ 13,847
Operating property and equipment, accumulated depreciation | 1,210,379 | 1,192,377
Finite-Lived intangible assets, accumulated amortization | $ 58,760 | $ 57,420
Shareholders’ equity: |  |
Common stock, par value (in dollars per share) | $ 0.50 | $ 0.50
Common stock, shares authorized (in shares) | 400,000,000 | 400,000,000
Common stock, shares outstanding (in shares) | 53,094,898 | 52,955,314
Preferred stock, par value (in dollars per share) | $ 0 | $ 0
Preferred stock, shares authorized (in shares) | 3,800,917 | 3,800,917
Preferred stock, shares outstanding (in shares) | 0 | 0

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