Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Trade accounts and notes receivable, allowance for doubtful accounts	$ 803	$ 860
Property, plant and equipment, accumulated depreciation	456,669	427,766
Intangible assets, Accumulated amortization	304,416	281,439
Unamortized discount and debt issuance costs	$ 8,627	$ 9,470
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	400,000,000	400,000,000
Common stock, shares issued	141,691,182	141,282,539
Common stock, shares outstanding	141,691,182	141,282,539