Condensed Consolidated Balance Sheets (unaudited) (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Accounts receivable, allowances for doubtful accounts	$ 14,953	$ 11,820	$ 13,014
Property, plant and equipment, accumulated depreciation and amortization	$ 1,524,282	$ 1,511,890	$ 1,527,769
Common unitholders, units issued (in shares)	92,963,340	92,958,586	92,926,920