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Investments - Additional Information (Detail) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | ---
Mar. 31, 2019 | Jun. 30, 2019 | Dec. 31, 2018 | Jan. 01, 2018
Schedule of Investments [Line Items] |  |  |  |
Restricted certificate of deposit held |  | $ 25,600 | $ 25,600 |
Transfer to investments | $ 3,400 |  |  |
Carrying value |  | $ 3,400 |  |
Other Receivables [Member] |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Note receivable |  |  | 4,300 |
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 |  | $ 25,600 | 25,600 |
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 |  | $ 25,600 | $ 25,400 |
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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