Benefit Plans - Fair Value Measurement (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	$ 210,508	$ 222,124	$ 206,639
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	$ 209,848	$ 221,131
Qualified Plan | Equity securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of total investments	28.00%	38.00%
Fair value of plan assets	$ 59,447	$ 84,791
Qualified Plan | Corporate bonds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of total investments	24.00%	22.00%
Fair value of plan assets	$ 50,889	$ 47,471
Qualified Plan | Mutual funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of total investments	17.00%	23.00%
Fair value of plan assets	$ 36,293	$ 48,814
Qualified Plan | U.S. Treasury securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of total investments	24.00%	13.00%
Fair value of plan assets	$ 50,838	$ 28,671
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	$ 7,429	$ 9,522
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%
Fair value of plan assets	$ 4,952	1,862
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	154,007	171,798
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	59,447	84,791
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	36,293	48,814
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	50,838	28,671
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	7,429	9,522
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	55,841	49,333
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) | Corporate bonds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	50,889	47,471
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) | 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) | 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	4,952	1,862
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) | 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) | 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) | 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