Investments - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Restricted certificate of deposit held	$ 27,800	$ 27,800
Identification of potential other-than-temporary impairments	25.00%
Alabama State Industrial Development Authority [Member]
Schedule of Investments [Line Items]
Maturity date of bond	Jan. 01, 2020
Percentage of interest on bond	2.00%
Alabama State Industrial Development Authority [Member] \| Taxable Revenue Bond [Member]
Schedule of Investments [Line Items]
Collateral deposit against outstanding principal amount	$ 27,800	27,800
Alabama State Industrial Development Authority [Member] \| Taxable Revenue Bond [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Schedule of Investments [Line Items]
Estimated fair value of bond	$ 28,000	$ 28,100
Investment [Member] \| Issuer Concentration [Member] \| Market Value of Total Investment Portfolio [Member]
Schedule of Investments [Line Items]
Investment concentration risk percentage	5.00%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
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-Section 50
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Amount of securities, mortgage loans, or other financial instruments pledged as collateral for deposits.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Investments which are not defined as or included in marketable (debt, equity, or other) securities whose use is restricted in whole or in part, generally by contractual agreements or regulatory requirements. For use in an unclassified balance sheet.

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Threshold percentage by which investments in equity securities have declined in value to be considered other than temporary impairment (OTTI).

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