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Commitments and Contingencies - Additional Information (Detail) |  |  | 9 Months Ended |  |
--- | --- | --- | --- | --- | ---
Aug. 30, 2021 shares | Mar. 31, 2020 Officer | Sep. 30, 2021 USD ($) | Mar. 31, 2021 USD ($) | Dec. 31, 2020 USD ($)
Contingencies And Commitments [Line Items] |  |  |  |  |
Commitments related to performance bonds |  |  | $ 20,900,000 |  | $ 15,200,000
Commitments related to performance bonds expiration month and year |  |  | 2024-08 |  |
Letter of credit secured by pledge of a portion of fixed income securities |  |  | $ 18,400,000 |  |
Commitments towards private equity funds |  |  | 4,900,000 |  |
Investment Commitments [Member] |  |  |  |  |
Contingencies And Commitments [Line Items] |  |  |  |  |
Aggregate investment committed in private equity funds |  |  | 5,000,000.0 |  |
Long-term Investments [Member] |  |  |  |  |
Contingencies And Commitments [Line Items] |  |  |  |  |
Letter of credit secured by pledge of a portion of fixed income securities |  |  | 18,300,000 |  |
Minimum [Member] |  |  |  |  |
Contingencies And Commitments [Line Items] |  |  |  |  |
Collateral value required to be maintained |  |  |  | $ 15,000,000.0 |
Maximum [Member] | Restricted Cash [Member] |  |  |  |  |
Contingencies And Commitments [Line Items] |  |  |  |  |
Letter of credit secured by pledge of a portion of fixed income securities |  |  | $ 100,000 |  |
Acorn HoldCo, Inc., [Member] |  |  |  |  |
Contingencies And Commitments [Line Items] |  |  |  |  |
Business combination date of agreement | Aug. 30, 2021 |  |  |  |
Acorn HoldCo, Inc., [Member] | ADTRAN INC [Member] |  |  |  |  |
Contingencies And Commitments [Line Items] |  |  |  |  |
Percentage of ownership interest acquired | 54.00% |  |  |  |
Acorn HoldCo, Inc., [Member] | ADTRAN INC [Member] | Common Stock [Member] |  |  |  |  |
Contingencies And Commitments [Line Items] |  |  |  |  |
Number of shares issuable for each share of acquired entity | shares | 1 |  |  |  |
Acorn HoldCo, Inc., [Member] | ADVA Optical Networking SE, [Member] |  |  |  |  |
Contingencies And Commitments [Line Items] |  |  |  |  |
Percentage of ownership interest acquired | 46.00% |  |  |  |
Acorn HoldCo, Inc., [Member] | ADVA Optical Networking SE, [Member] | Common Stock [Member] |  |  |  |  |
Contingencies And Commitments [Line Items] |  |  |  |  |
Number of shares issuable for each share of acquired entity | shares | 0.8244 |  |  |  |
Acorn HoldCo, Inc., [Member] | EGORA Holding GmbH and Egora Investments GmbH [Member] | Irrevocable Undertaking [Member] |  |  |  |  |
Contingencies And Commitments [Line Items] |  |  |  |  |
Number of shares issuable | shares | 7,000,000 |  |  |  |
Percentage of ownership interest acquired | 13.70% |  |  |  |
Current Executive Officers [Member] |  |  |  |  |
Contingencies And Commitments [Line Items] |  |  |  |  |
Number of officers | Officer |  | 2 |  |  |
Former Executive Officers [Member] |  |  |  |  |
Contingencies And Commitments [Line Items] |  |  |  |  |
Number of officers | Officer |  | 1 |  |  |

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