Debt - Debt (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total debt	$ 286,816	$ 290,799
Less current maturities	24,312	22,500
Long-term debt	262,504	268,299
Unsecured private placement 3.95% note [Member]
Debt Instrument [Line Items]
Total debt	$ 99,619	$ 99,654
Debt instrument interest rate percentage	3.95%	3.95%
Unsecured private placement 3.95% note [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Maturity Dates	2021	2021
Unsecured private placement 3.95% note [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Maturity Dates	2027	2027
Unsecured private placement 3.86% note [Member]
Debt Instrument [Line Items]
Total debt	$ 99,625	$ 99,657
Debt instrument interest rate percentage	3.86%	3.86%
Unsecured private placement 3.86% note [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Maturity Dates	2019	2019
Unsecured private placement 3.86% note [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Maturity Dates	2025	2025
Unsecured private placement 4.86% note [Member]
Debt Instrument [Line Items]
Total debt	$ 55,509	$ 55,523
Debt instrument interest rate percentage	4.86%	4.86%
Unsecured private placement 4.86% note [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Maturity Dates	2018	2018
Unsecured private placement 4.86% note [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Maturity Dates	2023	2023
Unsecured private placement 5.88% note [Member]
Debt Instrument [Line Items]
Total debt	$ 22,758	$ 28,476
Debt instrument interest rate percentage	5.88%	5.88%
Unsecured private placement 5.88% note [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Maturity Dates	2018	2018
Unsecured private placement 5.88% note [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Maturity Dates	2022	2022
Unsecured private placement 5.69% note [Member]
Debt Instrument [Line Items]
Total debt	$ 5,707	$ 5,703
Debt instrument interest rate percentage	5.69%	5.69%
Maturity Dates	2018	2018
Debt of foreign subsidiaries Unsecured bank debt, foreign currency [Member]
Debt Instrument [Line Items]
Total debt	$ 3,598	$ 1,786
Maturity Dates	2018	2018

Debt instruments maturity year.

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19,20)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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