Separate Financial Information of Guarantor Subsidiaries Statements of Cash Flows (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
OPERATING ACTIVITIES
Net Cash Provided by Operating Activities		$ 79,970	$ 54,174	$ 57,878
INVESTING ACTIVITIES
Purchases of Property, Plant and Equipment		(18,780)	(20,437)	(26,526)
Proceeds from Disposals of Property, Plant and Equipment		112	2,511	615
Proceeds from Principal Payments Received on Long-Term Note Receivable		1,416	667	0
Proceeds from Sale of Business	$ 4,000	4,000	0	285
Acquisitions of Businesses, Net of Cash Acquired		0	(354,073)	(12,933)
Issuance of Long-Term Note Receivable		0	(1,500)	(2,000)
Purchases of Intangible Asset		(2,775)	(2,500)	0
Change in Investments in Subsidiaries		0	0	0
Loan Proceeds Received by the Parent from Subsidiaries		0
Loan Proceeds Received by Subsidiaries from the Parent		0
Loan Borrowings (Payments) from Subsidiaries			0	0
Net Cash Used in Investing Activities		(16,027)	(375,332)	(40,559)
FINANCING ACTIVITIES
Proceeds from Short-Term Debt		3,926	303,000	0
Repayments of Debt from Subsidiaries to Parent		0
Repayments of Debt from the Parent to Subsidiaries		0
Repayments of Short-term Debt		0	(303,000)	0
Loan Borrowings (Payments) from Parent			0	0
Change in Subsidiary Equity		0	0	0
Proceeds from Issuance of Long-Term Debt		11,000	440,000	15,000
Payments of Long-Term Debt		(38,255)	(96,248)	(3,460)
Purchases of Common Stock		0	0	(12,762)
Payments of Debt Issuance Costs		0	(16,482)	0
Change in Capital Lease Obligations		14	311	0
Proceeds from Issuances of Common Stock		5,880	6,875	5,271
Payments to Noncontrolling Interests		0	(30)	0
Excess Tax Benefit on Stock Plans		0	0	686
Dividends Paid		(15,343)	(14,953)	(14,293)
Net Cash (Used in) Provided by Financing Activities		(32,778)	319,473	(9,558)
Effect of Exchange Rate on Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents		(4,082)	2,186	(1,150)
NET INCREASE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH		27,083	501	6,611
Cash, Cash Equivalents and Restricted Cash at Beginning of Year	59,051	59,051	58,550	51,939
CASH, CASH EQUIVALENTS, RESTRICTED CASH AND RESTRICTED CASH EQUIVALENTS AT END OF YEAR		86,134	59,051	58,550
Parent Company
OPERATING ACTIVITIES
Net Cash Provided by Operating Activities		68,082	26,992	44,147
INVESTING ACTIVITIES
Purchases of Property, Plant and Equipment		(6,832)	(9,558)	(21,507)
Proceeds from Disposals of Property, Plant and Equipment		21	23	377
Proceeds from Principal Payments Received on Long-Term Note Receivable		0	0
Proceeds from Sale of Business		0		0
Acquisitions of Businesses, Net of Cash Acquired			(304)	0
Issuance of Long-Term Note Receivable			0	0
Purchases of Intangible Asset		(2,500)	(2,500)
Change in Investments in Subsidiaries		(15,622)	(199,028)	(19,594)
Loan Proceeds Received by the Parent from Subsidiaries		1,218
Loan Proceeds Received by Subsidiaries from the Parent		0
Loan Borrowings (Payments) from Subsidiaries			(159,780)	8,690
Net Cash Used in Investing Activities		(23,715)	(371,147)	(32,034)
FINANCING ACTIVITIES
Proceeds from Short-Term Debt		0	303,000
Repayments of Debt from Subsidiaries to Parent		0
Repayments of Debt from the Parent to Subsidiaries		(1,778)
Repayments of Short-term Debt			(303,000)
Loan Borrowings (Payments) from Parent			4,983	0
Change in Subsidiary Equity		0	0
Proceeds from Issuance of Long-Term Debt		11,000	440,000	15,000
Payments of Long-Term Debt		(38,000)	(96,142)	(3,429)
Purchases of Common Stock				(12,762)
Payments of Debt Issuance Costs			(16,482)
Change in Capital Lease Obligations		0	0
Proceeds from Issuances of Common Stock		5,880	6,875	5,271
Payments to Noncontrolling Interests			0
Excess Tax Benefit on Stock Plans				686
Dividends Paid		(15,343)	(14,953)	(14,293)
Net Cash (Used in) Provided by Financing Activities		(38,241)	324,281	(9,527)
Effect of Exchange Rate on Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents		184	(141)	63
NET INCREASE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH		6,310	(20,015)	2,649
Cash, Cash Equivalents and Restricted Cash at Beginning of Year	18,469	18,469	38,484	35,835
CASH, CASH EQUIVALENTS, RESTRICTED CASH AND RESTRICTED CASH EQUIVALENTS AT END OF YEAR		24,779	18,469	38,484
Guarantor Subsidiaries
OPERATING ACTIVITIES
Net Cash Provided by Operating Activities		1,202	280	239
INVESTING ACTIVITIES
Purchases of Property, Plant and Equipment		(99)	0	(13)
Proceeds from Disposals of Property, Plant and Equipment		0	1	0
Proceeds from Principal Payments Received on Long-Term Note Receivable		0	0
Proceeds from Sale of Business		0		0
Acquisitions of Businesses, Net of Cash Acquired			0	(11,539)
Issuance of Long-Term Note Receivable			0	0
Purchases of Intangible Asset		0	0
Change in Investments in Subsidiaries		0	0	0
Loan Proceeds Received by the Parent from Subsidiaries		0
Loan Proceeds Received by Subsidiaries from the Parent		0
Loan Borrowings (Payments) from Subsidiaries			0	0
Net Cash Used in Investing Activities		(99)	1	(11,552)
FINANCING ACTIVITIES
Proceeds from Short-Term Debt		0	0
Repayments of Debt from Subsidiaries to Parent		0
Repayments of Debt from the Parent to Subsidiaries		0
Repayments of Short-term Debt			0
Loan Borrowings (Payments) from Parent			0	7,969
Change in Subsidiary Equity		0	0	3,570
Proceeds from Issuance of Long-Term Debt		0	0	0
Payments of Long-Term Debt		0	0	0
Purchases of Common Stock				0
Payments of Debt Issuance Costs			0
Change in Capital Lease Obligations		0	0
Proceeds from Issuances of Common Stock		0	0	0
Payments to Noncontrolling Interests			0
Excess Tax Benefit on Stock Plans				0
Dividends Paid		0	0	0
Net Cash (Used in) Provided by Financing Activities		0	0	11,539
Effect of Exchange Rate on Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents		0	0	0
NET INCREASE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH		1,103	281	226
Cash, Cash Equivalents and Restricted Cash at Beginning of Year	507	507	226	0
CASH, CASH EQUIVALENTS, RESTRICTED CASH AND RESTRICTED CASH EQUIVALENTS AT END OF YEAR		1,610	507	226
Non-Guarantor Subsidiaries
OPERATING ACTIVITIES
Net Cash Provided by Operating Activities		10,888	27,711	14,090
INVESTING ACTIVITIES
Purchases of Property, Plant and Equipment		(11,849)	(10,879)	(5,006)
Proceeds from Disposals of Property, Plant and Equipment		91	2,487	238
Proceeds from Principal Payments Received on Long-Term Note Receivable		1,416	667
Proceeds from Sale of Business		4,000		285
Acquisitions of Businesses, Net of Cash Acquired			(353,769)	(1,394)
Issuance of Long-Term Note Receivable			(1,500)	(2,000)
Purchases of Intangible Asset		(275)	0
Change in Investments in Subsidiaries		0	0	0
Loan Proceeds Received by the Parent from Subsidiaries		0
Loan Proceeds Received by Subsidiaries from the Parent		1,778
Loan Borrowings (Payments) from Subsidiaries			(4,983)	0
Net Cash Used in Investing Activities		(4,839)	(367,977)	(7,877)
FINANCING ACTIVITIES
Proceeds from Short-Term Debt		3,926	0
Repayments of Debt from Subsidiaries to Parent		(1,218)
Repayments of Debt from the Parent to Subsidiaries		0
Repayments of Short-term Debt			0
Loan Borrowings (Payments) from Parent			159,780	(16,659)
Change in Subsidiary Equity		15,622	199,028	16,024
Proceeds from Issuance of Long-Term Debt		0	0	0
Payments of Long-Term Debt		(255)	(106)	(31)
Purchases of Common Stock				0
Payments of Debt Issuance Costs			0
Change in Capital Lease Obligations		14	311
Proceeds from Issuances of Common Stock		0	0	0
Payments to Noncontrolling Interests			(30)
Excess Tax Benefit on Stock Plans				0
Dividends Paid		(202)	(809)	(598)
Net Cash (Used in) Provided by Financing Activities		17,887	358,174	(1,264)
Effect of Exchange Rate on Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents		(4,266)	2,327	(1,213)
NET INCREASE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH		19,670	20,235	3,736
Cash, Cash Equivalents and Restricted Cash at Beginning of Year	40,075	40,075	19,840	16,104
CASH, CASH EQUIVALENTS, RESTRICTED CASH AND RESTRICTED CASH EQUIVALENTS AT END OF YEAR		59,745	40,075	19,840
Eliminations
OPERATING ACTIVITIES
Net Cash Provided by Operating Activities		(202)	(809)	(598)
INVESTING ACTIVITIES
Purchases of Property, Plant and Equipment		0	0	0
Proceeds from Disposals of Property, Plant and Equipment		0	0	0
Proceeds from Principal Payments Received on Long-Term Note Receivable		0	0
Proceeds from Sale of Business		0		0
Acquisitions of Businesses, Net of Cash Acquired			0	0
Issuance of Long-Term Note Receivable			0	0
Purchases of Intangible Asset		0	0
Change in Investments in Subsidiaries		15,622	199,028	19,594
Loan Proceeds Received by the Parent from Subsidiaries		(1,218)
Loan Proceeds Received by Subsidiaries from the Parent		(1,778)
Loan Borrowings (Payments) from Subsidiaries			164,763	(8,690)
Net Cash Used in Investing Activities		12,626	363,791	10,904
FINANCING ACTIVITIES
Proceeds from Short-Term Debt		0	0
Repayments of Debt from Subsidiaries to Parent		1,218
Repayments of Debt from the Parent to Subsidiaries		1,778
Repayments of Short-term Debt			0
Loan Borrowings (Payments) from Parent			(164,763)	8,690
Change in Subsidiary Equity		(15,622)	(199,028)	(19,594)
Proceeds from Issuance of Long-Term Debt		0	0	0
Payments of Long-Term Debt		0	0	0
Purchases of Common Stock				0
Payments of Debt Issuance Costs			0
Change in Capital Lease Obligations		0	0
Proceeds from Issuances of Common Stock		0	0	0
Payments to Noncontrolling Interests			0
Excess Tax Benefit on Stock Plans				0
Dividends Paid		202	809	598
Net Cash (Used in) Provided by Financing Activities		(12,424)	(362,982)	(10,306)
Effect of Exchange Rate on Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents		0	0	0
NET INCREASE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH		0	0	0
Cash, Cash Equivalents and Restricted Cash at Beginning of Year	$ 0	0	0	0
CASH, CASH EQUIVALENTS, RESTRICTED CASH AND RESTRICTED CASH EQUIVALENTS AT END OF YEAR		$ 0	$ 0	$ 0