Benefit Plans - Fair Value Measurement (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	$ 260,651	$ 236,621	$ 210,508
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	$ 260,028	$ 235,981
Qualified Plan | Equity securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of total investments	37.00%	32.00%
Fair value of plan assets	$ 94,434	$ 75,633
Qualified Plan | U.S. Treasury securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of total investments	20.00%	22.00%
Fair value of plan assets	$ 52,549	$ 51,732
Qualified Plan | Corporate bonds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of total investments	20.00%	21.00%
Fair value of plan assets	$ 51,410	$ 51,221
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	$ 48,041	$ 42,119
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%	3.00%
Fair value of plan assets	$ 8,174	$ 6,263
Qualified Plan | Cash and money market funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Percent of total investments	2.00%	4.00%
Fair value of plan assets	$ 5,420	$ 9,013
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	200,444	178,497
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	94,434	75,633
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	52,549	51,732
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	48,041	42,119
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 | 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	5,420	9,013
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	59,584	57,484
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	51,410	51,221
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. government agency securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value of plan assets	8,174	6,263
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 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) | 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 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