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Employee Benefits - Fair Value Measurements of Investments Held by Retirement Plan (Detail) $ in Thousands | Dec. 31, 2020 USD ($)
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Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | $ 261,030 |
Common Stock | Equity Securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 31,401 |
Money Market | Cash Equivalents |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 5,237 |
Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 33,722 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Common Stock | Equity Securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 31,401 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money Market | Cash Equivalents |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 2,321 |
Significant Other Observable Inputs (Level 2) |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 227,308 |
Significant Other Observable Inputs (Level 2) | Money Market | Cash Equivalents |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 2,916 |
Large-Cap Value | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 23,206 | [1]
Large-Cap Value | Significant Other Observable Inputs (Level 2) | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 23,206 | [1]
Large-Cap Growth | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 22,997 | [2]
Large-Cap Growth | Significant Other Observable Inputs (Level 2) | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 22,997 | [2]
Large-Cap Core | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 15,146 | [3]
Large-Cap Core | Significant Other Observable Inputs (Level 2) | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 15,146 | [3]
United States Mid Cap Value | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 4,509 | [4]
United States Mid Cap Value | Significant Other Observable Inputs (Level 2) | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 4,509 | [4]
Mid Cap Growth | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 6,316 | [5]
Mid Cap Growth | Significant Other Observable Inputs (Level 2) | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 6,316 | [5]
Mid Cap Core | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 5,113 | [6]
Mid Cap Core | Significant Other Observable Inputs (Level 2) | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 5,113 | [6]
US Small-Cap Value | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 3,307 | [7]
US Small-Cap Value | Significant Other Observable Inputs (Level 2) | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 3,307 | [7]
Small Cap Value Growth | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 8,674 | [8]
Small Cap Value Growth | Significant Other Observable Inputs (Level 2) | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 8,674 | [8]
Small-Cap Core | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 3,400 | [9]
Small-Cap Core | Significant Other Observable Inputs (Level 2) | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 3,400 | [9]
International Equity | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 32,248 | [10]
International Equity | Significant Other Observable Inputs (Level 2) | Equity Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 32,248 | [10]
U.S. Core | Fixed Income Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 74,523 | [11]
U.S. Core | Significant Other Observable Inputs (Level 2) | Fixed Income Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 74,523 | [11]
Intermediate Duration | Fixed Income Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 24,953 | [12]
Intermediate Duration | Significant Other Observable Inputs (Level 2) | Fixed Income Funds |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | $ 24,953 | [12]
[1] | This category contains large-cap stocks with above-average yield. The portfolio typically holds between 60 and 70 stocks.
[2] | This category seeks long-term capital appreciation by investing primarily in large growth companies based in the U.S.
[3] | This fund tracks the performance of the S&P 500 Index by purchasing the securities represented in the Index in approximately the same weightings as the Index.
[4] | This category employs an indexing investment approach designed to track the performance of the CRSP US Mid-Cap Value Index.
[5] | This category employs an indexing investment approach designed to track the performance of the CRSP US Mid-Cap Growth Index.
[6] | This category seeks to track the performance of the S&P Midcap 400 Index.
[7] | This category consists of a selection of investments based on the Russell 2000 Value Index.
[8] | This category consists of a mutual fund invested in small cap growth companies along with a fund invested in a selection of investments based on the Russell 2000 Growth Index.
[9] | This category consists of a mutual fund investing in readily marketable securities of U.S. companies with market capitalizations within the smallest 10% of the market universe, or smaller than the 1000th largest US company.
[10] | This category invests primarily in medium to large non-US companies in developed and emerging markets. Under normal circumstances, at least 80% of total assets will be invested in equity securities, including common stocks, preferred stocks, and convertible securities.
[11] | This category currently includes equal investments in three mutual funds, two of which usually hold at least 80% of fund assets in investment grade fixed income securities, seeking to outperform the Barclays US Aggregate Bond Index while maintaining a similar duration to that index. The third fund targets investments of 50% or more in mortgage-backed securities guaranteed by the US government and its agencies
[12] | This category consists of a mutual fund which invest in a diversified portfolio of high-quality bonds and other fixed income securities, including U.S. Government obligations, mortgage-related and asset backed securities, corporate and municipal bonds, CMOs, and other securities mostly rated A or better.

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