Consolidated Condensed Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets:
Allowance for doubtful accounts, current	$ 18,802	$ 17,182
Operating property and equipment, accumulated depreciation	1,260,292	1,256,037
Finite-Lived intangible assets, accumulated amortization	$ 69,236	$ 65,048
Shareholders’ equity:
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
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)	53,334,512	53,116,485