Long-Term Debt (Details) - USD ($)	Sep. 30, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Unamortized Debt Issuance Expense		$ (16,800,000)
Capital leases	$ 57,000	83,000
Total long-term debt	1,030,165,000	1,057,847,000
less: current portion of long-term debt	25,000	31,000
Long-term debt, net	1,030,140,000	1,057,816,000
Senior Unsecured Notes due 2021 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	300,000,000	300,000,000
Unamortized Debt Issuance Expense	(3,917,000)	(4,540,000)
Senior Notes	296,083,000	295,460,000
Senior Unsecured Notes due 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	400,000,000	400,000,000
Unamortized Debt Issuance Expense	(6,175,000)	(6,975,000)
Senior Notes	393,825,000	393,025,000
Senior unsecured notes due 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	350,000,000	350,000,000
Debt Instrument, Unamortized Discount	(5,036,000)	(5,455,000)
Unamortized Debt Issuance Expense	(4,764,000)	(5,266,000)
Senior Notes	340,200,000	339,279,000
SemMexico [Member]
Debt Instrument [Line Items]
Borrowings	0	0
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Borrowings	0
Revolving Credit Facility [Member] \| Corporate, Non-Segment [Member]
Debt Instrument [Line Items]
Borrowings	$ 0	$ 30,000,000

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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