Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 4,559	$ 3,627
Accumulated depreciation on property, plant and equipment	1,626,778	1,589,422
Accumulated amortization on other assets	$ 3,871	$ 3,054