Supplemental Information (Cash Flow Information) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Increase (decrease) in capital expenditure accrual	$ 867	$ 18,113	$ (14,552)
Interest Paid, Net	23,063	9,901	9,538
Income Taxes Paid	716	633	1,264
State and Local Jurisdiction [Member]
Related Party Transaction [Line Items]
Income Taxes Paid	301	388	231
Reimbursement to Parent for State Tax [Member] | Westlake [Member] | Affiliated Entity [Member]
Related Party Transaction [Line Items]
Income Taxes Paid	$ 415	$ 245	$ 1,033