Consolidated Condensed Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets:
Allowance for doubtful accounts, current	$ 15,648	$ 13,847
Operating property and equipment, accumulated depreciation	1,224,530	1,192,377
Finite-Lived intangible assets, accumulated amortization	$ 60,800	$ 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)	53,094,736	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