Long-Term Debt Long-Term Debt (Details 3) $ in Thousands, MXN in Millions	Dec. 31, 2017 USD ($)	Dec. 31, 2017 MXN
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate at Period End	2.50%	2.50%
Letters of credit outstanding	$ 39,385
Bilateral Letter of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate at Period End	1.75%	1.75%
Letters of credit outstanding	$ 56,525
SemMexico [Member] | Letter of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate at Period End	0.28%	0.28%
Letters of credit outstanding	$ 14,870	MXN 292.8