Long-Term Debt (Details 1) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Capital leases	$ 51	$ 83
Total long-term debt	1,050,944	1,057,847
less: current portion of long-term debt	26	31
Noncurrent portion of long-term debt	1,050,918	1,057,816
Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	1,050,000
Senior Notes [Member] \| Senior Unsecured Notes due 2021 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	300,000	300,000
Debt Issuance Costs, Net	3,708	4,540
Long-term Debt	296,292	295,460
Senior Notes [Member] \| Senior unsecured notes due 2022 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	400,000	400,000
Debt Issuance Costs, Net	5,909	6,975
Long-term Debt	394,091	393,025
Senior Notes [Member] \| Senior unsecured notes due 2023 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	350,000	350,000
Debt Issuance Costs, Net	4,596	5,266
Long-term Debt	340,510	339,279
Debt Instrument, Unamortized Discount	(4,894)	(5,455)
Line of Credit [Member] \| Revolving Credit Facility [Member] \| Corporate Credit Facility [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	20,000	30,000
Line of Credit [Member] \| Revolving Credit Facility [Member] \| SemMexico Credit Facility [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 0	$ 0

Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.

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