Consolidated Condensed Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Assets:
Allowance for doubtful accounts, current	$ 12,210	$ 14,915
Operating property and equipment, accumulated depreciation	1,144,914	1,128,040
Finite-Lived intangible assets, accumulated amortization	$ 53,022	$ 51,578
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)	53,560,199	53,463,118
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