Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Allowance for Doubtful Accounts Receivable, Current	$ 1,788	$ 2,628
Accumulated depreciation	483,904	444,842
Accumulated amortization	$ 64,810	$ 56,409
Preferred Stock, Par or Stated Value Per Share	$ 0.01	$ 0
Preferred Stock, Liquidation Preference, Value	$ 350,000	$ 0
Preferred Stock, Shares Authorized	4,000,000	0
Preferred Stock, Shares Issued	350,000	0
Accounts Payable	$ 492,507	$ 587,898
Accrued Liabilities	100,373	131,407
Other current liabilities	$ 19,184	$ 3,395
Common stock, $0.01 par value	$ 0.01	$ 0.01
Common stock shares authorized	100,000,000	100,000,000
Common stock shares issued	79,062,000	79,708,000
Treasury Stock, Common, Shares	35,000	1,024,000