GRT Acquisition - Estimated Fair Values of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	16 Months Ended
	Mar. 31, 2018	Jun. 30, 2018	Dec. 31, 2017	Dec. 01, 2016
Assets
Goodwill		$ 53,960	$ 54,613
GRT Acquisition
Assets
Cash	$ 8,091			$ 8,091
Accounts receivable	2,844			2,844
Notes receivable	3,050			3,050
Inventories	8,468			7,983
Other current assets	981			981
Property, plant and equipment	47,541			46,712
Intangible assets	7,428			7,412
Other long-term assets	289			289
Goodwill	33,250			33,861
Liabilities
Accounts payable	(62,289)			(61,570)
Notes payable	(10,122)			(10,122)
Accrued liabilities	(2,866)			(2,866)
Long-term debt	(3,383)			(3,383)
Other long-term liabilities	(940)			(940)
Net assets and liabilities assumed	32,342			32,342
Noncontrolling shareholder interest	(18,323)			(18,323)
Cooper Tire & Rubber Company consideration	14,019			$ 14,019
Adjustments
Inventories	485
Property, plant and equipment	829
Intangible assets	16
Goodwill	(611)
Accounts payable	$ (719)