Benefit Plans - Fair Value Measurement of Plan Assets (Details) - Pension Benefits - USD ($)	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Benefit Plans
Fair value of plan assets	$ 114,795,000	$ 112,659,000	$ 99,581,000
Percentage of total	100.00%	100.00%
Minimum
Benefit Plans
Percentage of assets in high-grade corporate bonds and U.S. government debt obligations	80.00%
Cash and cash equivalents
Benefit Plans
Fair value of plan assets	$ 2,576,000	$ 2,824,000
Fixed income securities
Benefit Plans
Percentage of total	85.10%	66.20%
U.S. Treasury securities
Benefit Plans
Fair value of plan assets	$ 6,776,000	$ 4,053,000
Government agency debt securities
Benefit Plans
Fair value of plan assets	12,441,000	3,504,000
Fixed income - U.S. corporate securities
Benefit Plans
Fair value of plan assets	70,401,000	58,808,000
Fixed income - municipal securities
Benefit Plans
Fair value of plan assets	521,000	484,000
Fixed income - mutual funds
Benefit Plans
Fair value of plan assets	5,626,000	6,204,000
Fixed income - international securities
Benefit Plans
Fair value of plan assets	1,980,000	1,544,000
Equity - large-cap exchange-traded fund
Benefit Plans
Fair value of plan assets	9,321,000	23,278,000
Equity - mid-cap exchange-traded funds
Benefit Plans
Fair value of plan assets	1,566,000	3,379,000
Equity - small-cap exchange-traded funds
Benefit Plans
Fair value of plan assets	785,000	1,645,000
Equity - international funds
Benefit Plans
Fair value of plan assets	2,802,000	6,936,000
Level 1
Benefit Plans
Fair value of plan assets	24,656,000	45,810,000
Level 1 | Cash and cash equivalents
Benefit Plans
Fair value of plan assets	2,576,000	2,824,000
Level 1 | Fixed income - mutual funds
Benefit Plans
Fair value of plan assets	5,626,000	6,204,000
Level 1 | Fixed income - international securities
Benefit Plans
Fair value of plan assets	1,980,000	1,544,000
Level 1 | Equity - large-cap exchange-traded fund
Benefit Plans
Fair value of plan assets	9,321,000	23,278,000
Level 1 | Equity - mid-cap exchange-traded funds
Benefit Plans
Fair value of plan assets	1,566,000	3,379,000
Level 1 | Equity - small-cap exchange-traded funds
Benefit Plans
Fair value of plan assets	785,000	1,645,000
Level 1 | Equity - international funds
Benefit Plans
Fair value of plan assets	2,802,000	6,936,000
Level 2
Benefit Plans
Fair value of plan assets	90,139,000	66,849,000
Level 2 | U.S. Treasury securities
Benefit Plans
Fair value of plan assets	6,776,000	4,053,000
Level 2 | Government agency debt securities
Benefit Plans
Fair value of plan assets	12,441,000	3,504,000
Level 2 | Fixed income - U.S. corporate securities
Benefit Plans
Fair value of plan assets	70,401,000	58,808,000
Level 2 | Fixed income - municipal securities
Benefit Plans
Fair value of plan assets	521,000	484,000
Level 3
Benefit Plans
Fair value of plan assets	$ 0	$ 0