Financial Instruments (Bonds) - Additional Information (Detail) - USD ($)	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Derivatives Fair Value [Line Items]
Corporate and municipal bonds	$ 51,500,000	$ 49,500,000
Maturity dates of bonds, start date	2018-10
Maturity dates of bonds, end date	2023-11
Interest Rate Swap [Member]
Derivatives Fair Value [Line Items]
Interest rate swap, description of interest received	interest that varies with the one-month LIBOR
Derivative, Type of Interest Rate Paid on Swap	fixed
Interest Rate Swap [Member] \| November 2018 [Member]
Derivatives Fair Value [Line Items]
Notional value	$ 15,000,000
Interest rate swap, fixed interest rate	1.155%
Interest Rate Swap [Member] \| October 2020 [Member]
Derivatives Fair Value [Line Items]
Notional value	$ 25,000,000
Interest rate swap, fixed interest rate	1.30%
Interest Rate Swap [Member] \| February 2021 [Member]
Derivatives Fair Value [Line Items]
Notional value	$ 10,000,000
Interest rate swap, fixed interest rate	1.12%
Interest Rate Swap [Member] \| May 2022 [Member]
Derivatives Fair Value [Line Items]
Notional value	$ 20,000,000
Interest rate swap, fixed interest rate	1.77%
Interest Rate Swap [Member] \| June 2023 [Member]
Derivatives Fair Value [Line Items]
Notional value	$ 15,000,000
Interest rate swap, fixed interest rate	2.64%

Corporate and municipal bonds par value.

No definition available.

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Balance Type:	debit
Period Type:	instant

- Definition

Derivative Floating Interest Rate Description

+ References

No definition available.

+ Details

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- Definition

Maturity date of bond end date.

+ References

No definition available.

+ Details

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- Definition

Maturity date of bond start date.

+ References

No definition available.

+ Details

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- Definition

When the derivative or group of derivatives presented is a swap, the type of interest rate (fixed, variable, stepped, etc.) paid on the swap.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column A
-Footnote 1

+ Details

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- Definition

Fixed interest rate related to the interest rate derivative.

+ References

No definition available.

+ Details

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- Definition

Nominal or face amount used to calculate payments on the derivative liability.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5580258-113959

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

+ Details

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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.

+ Details

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- Details

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- Details

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