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Investments - Additional Information (Detail) - USD ($) $ in Thousands | 6 Months Ended |  |
--- | --- | --- | ---
Jun. 30, 2018 | Jan. 01, 2018 | Dec. 31, 2017
Schedule of Investments [Line Items] |  |  |
Restricted certificate of deposit held | $ 26,700 |  | $ 27,800
ASU 2016-01 [Member] |  |  |
Schedule of Investments [Line Items] |  |  |
Reclassification of net unrealized gains of marketable equity securities from AOCI to retained earnings |  | $ 3,200 |
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 | $ 26,700 |  | 26,700
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 | $ 26,500 |  | $ 26,700
Investment [Member] | Issuer Concentration [Member] | Market Value of Total Investment Portfolio [Member] |  |  |
Schedule of Investments [Line Items] |  |  |
Investment concentration risk percentage | 5.00% |  |

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