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CONDENSED CONSOLIDATED BALANCE SHEETS (PARENTHETICAL) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Statement of Financial Position [Abstract]    
Allowance for doubtful accounts $ 26,560 $ 29,267
Cumulative preferred stock, par value (in dollars per share) $ 5 $ 5
Cumulative preferred stock, shares authorized 12,000,000 12,000,000
Cumulative preferred stock, shares issued 0 0
Cumulative preferred stock, shares outstanding 0 0
Common stock, par value (in dollars per share) $ 0.5 $ 0.5
Common stock, shares authorized 300,000,000 300,000,000
Common stock, shares issued 109,659,219 109,659,219
Treasury stock, shares at cost 53,525,404 53,330,356