Benefit Plans - Fair Value Measurement (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	$ 236,621	$ 210,508	$ 222,124
Qualified Plan
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of total investments	100.00%	100.00%
Fair value of plan assets	$ 235,981	$ 209,847
Qualified Plan | Equity securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of total investments	32.00%	28.00%
Fair value of plan assets	$ 75,633	$ 59,447
Qualified Plan | U.S. Treasury securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of total investments	22.00%	24.00%
Fair value of plan assets	$ 51,732	$ 50,838
Qualified Plan | Corporate bonds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of total investments	21.00%	24.00%
Fair value of plan assets	$ 51,221	$ 50,003
Qualified Plan | Mutual funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of total investments	18.00%	18.00%
Fair value of plan assets	$ 42,119	$ 37,178
Qualified Plan | Cash and money market funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of total investments	4.00%	4.00%
Fair value of plan assets	$ 9,013	$ 7,429
Qualified Plan | U.S. government agency securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of total investments	3.00%	2.00%
Fair value of plan assets	$ 6,263	$ 4,952
Qualified Plan | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	178,497	154,892
Qualified Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	75,633	59,447
Qualified Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	51,732	50,838
Qualified Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	0	0
Qualified Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mutual funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	42,119	37,178
Qualified Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and money market funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	9,013	7,429
Qualified Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. government agency securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	0	0
Qualified Plan | Significant Other Observable Inputs (Level 2)
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	57,484	54,955
Qualified Plan | Significant Other Observable Inputs (Level 2) | Equity securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	0	0
Qualified Plan | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	0	0
Qualified Plan | Significant Other Observable Inputs (Level 2) | Corporate bonds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	51,221	50,003
Qualified Plan | Significant Other Observable Inputs (Level 2) | Mutual funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	0	0
Qualified Plan | Significant Other Observable Inputs (Level 2) | Cash and money market funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	0	0
Qualified Plan | Significant Other Observable Inputs (Level 2) | U.S. government agency securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	6,263	4,952
Qualified Plan | Significant Unobservable Inputs (Level 3)
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	0	0
Qualified Plan | Significant Unobservable Inputs (Level 3) | Equity securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	0	0
Qualified Plan | Significant Unobservable Inputs (Level 3) | U.S. Treasury securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	0	0
Qualified Plan | Significant Unobservable Inputs (Level 3) | Corporate bonds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	0	0
Qualified Plan | Significant Unobservable Inputs (Level 3) | Mutual funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	0	0
Qualified Plan | Significant Unobservable Inputs (Level 3) | Cash and money market funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	0	0
Qualified Plan | Significant Unobservable Inputs (Level 3) | U.S. government agency securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	$ 0	$ 0