Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets:
Allowance for doubtful accounts	$ 52,013	$ 51,501
Accumulated depreciation on property, plant and equipment	$ 989,117	$ 956,634
Shareholders’ equity:
Common shares, par value (in dollars per share)	$ 1.25	$ 1.25
Common shares, shares authorized (in shares)	305,000,000	305,000,000
Common shares, shares issued (in shares)	176,793,000	176,793,000
Treasury shares, shares (in shares)	46,053,000	46,237,000