Fair Value Measurements of Investments Held by Retirement Plan (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
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.