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Fair Value Measurements of Investments Held by Retirement Plan (Detail) $ in Thousands | Dec. 31, 2017 USD ($)
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Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | $ 234,136 |
Mutual Funds - Equity | Large-Cap Value |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 20,959 | [1]
Mutual Funds - Equity | Large-Cap Growth |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 21,825 | [2]
Mutual Funds - Equity | Large-Cap Core |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 14,512 | [3]
Mutual Funds - Equity | United States Mid Cap Value |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 4,668 | [4]
Mutual Funds - Equity | Mid Cap Growth |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 4,422 | [5]
Mutual Funds - Equity | Mid Cap Core |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 4,744 | [6]
Mutual Funds - Equity | US Small-Cap Value |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 3,530 | [7]
Mutual Funds - Equity | Small Cap Value Growth |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 3,353 | [8]
Mutual Funds - Equity | Small-Cap Core |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 6,908 | [9]
Mutual Funds - Equity | International Equity |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 28,113 | [10]
Fixed Income Funds | U.S. Core |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 68,928 | [11]
Fixed Income Funds | Intermediate Duration |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 23,046 | [12]
Equity securities | Common stock |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 24,865 |
Cash Equivalents | Money Market |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 4,263 | [13]
Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 25,928 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities | Common stock |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 24,865 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash Equivalents | Money Market |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 1,063 | [13]
Significant Other Observable Inputs (Level 2) |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 208,208 |
Significant Other Observable Inputs (Level 2) | Mutual Funds - Equity | Large-Cap Value |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 20,959 | [1]
Significant Other Observable Inputs (Level 2) | Mutual Funds - Equity | Large-Cap Growth |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 21,825 | [2]
Significant Other Observable Inputs (Level 2) | Mutual Funds - Equity | Large-Cap Core |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 14,512 | [3]
Significant Other Observable Inputs (Level 2) | Mutual Funds - Equity | United States Mid Cap Value |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 4,668 | [4]
Significant Other Observable Inputs (Level 2) | Mutual Funds - Equity | Mid Cap Growth |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 4,422 | [5]
Significant Other Observable Inputs (Level 2) | Mutual Funds - Equity | Mid Cap Core |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 4,744 | [6]
Significant Other Observable Inputs (Level 2) | Mutual Funds - Equity | US Small-Cap Value |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 3,530 | [7]
Significant Other Observable Inputs (Level 2) | Mutual Funds - Equity | Small Cap Value Growth |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 3,353 | [8]
Significant Other Observable Inputs (Level 2) | Mutual Funds - Equity | Small-Cap Core |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 6,908 | [9]
Significant Other Observable Inputs (Level 2) | Mutual Funds - Equity | International Equity |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 28,113 | [10]
Significant Other Observable Inputs (Level 2) | Fixed Income Funds | U.S. Core |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 68,928 | [11]
Significant Other Observable Inputs (Level 2) | Fixed Income Funds | Intermediate Duration |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | 23,046 | [12]
Significant Other Observable Inputs (Level 2) | Cash Equivalents | Money Market |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of assets | $ 3,200 | [13]
[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 selection of investments based on the Russell 2000 Growth Index.
[9] | This category consists of an index fund designed to track the Russell 2000, along with a 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 has investments in medium to large non-US companies, including high quality, durable growth companies and companies based in countries with stable economic and political systems. A portion of this category consists of an index fund designed to track the MSC ACWI ex-US Net Dividend Return Index.
[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.
[13] | Includes cash equivalent investments in equity and fixed income strategies.

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