Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Allowance for doubtful accounts receivable	$ 22,166	$ 17,612
Accumulated depreciation	$ 1,083,696	$ 1,044,353
Preferred stock, par value (usd per share)	$ 1	$ 1
Preferred stock, shares authorized (in shares)	10,000,000	10,000,000
Preferred stock, shares issued (in shares)	0	0
Common stock, par value (usd per share)	$ 1	$ 1
Common stock, shares authorized (in shares)	450,000,000	450,000,000
Common stock, shares issued (in shares)	146,737,803	146,652,515
Common stock, shares outstanding (in shares)	146,737,803	146,652,515