Benefit Plans - Fair Value Measurement of Plan Assets (Details) - Pension Benefits - USD ($)	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Benefit plans
Fair value of plan assets	$ 112,659,000	$ 99,581,000	$ 114,220,000
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	$ 2,824,000	$ 2,428,000
Fixed income securities
Benefit plans
Percentage of total	66.20%	68.70%
U.S. Treasury securities
Benefit plans
Fair value of plan assets	$ 4,053,000	$ 4,450,000
Government agency debt securities
Benefit plans
Fair value of plan assets	3,504,000	3,565,000
Fixed income - U.S. corporate securities
Benefit plans
Fair value of plan assets	58,808,000	53,721,000
Fixed income - municipal securities
Benefit plans
Fair value of plan assets	484,000	451,000
Fixed income - mutual funds
Benefit plans
Fair value of plan assets	6,204,000	6,272,000
Fixed income - international securities
Benefit plans
Fair value of plan assets	1,544,000
Equity - large-cap mutual funds
Benefit plans
Fair value of plan assets		8,831,000
Equity - large-cap exchange-traded fund
Benefit plans
Fair value of plan assets	23,278,000	10,482,000
Equity - mid-cap exchange-traded funds
Benefit plans
Fair value of plan assets	3,379,000	2,544,000
Equity - small-cap exchange-traded funds
Benefit plans
Fair value of plan assets	1,645,000	1,176,000
Equity - international funds
Benefit plans
Fair value of plan assets	6,936,000	5,661,000
Level 1
Benefit plans
Fair value of plan assets	45,810,000	37,394,000
Level 1 | Cash and cash equivalents
Benefit plans
Fair value of plan assets	2,824,000	2,428,000
Level 1 | Fixed income - mutual funds
Benefit plans
Fair value of plan assets	6,204,000	6,272,000
Level 1 | Fixed income - international securities
Benefit plans
Fair value of plan assets	1,544,000
Level 1 | Equity - large-cap mutual funds
Benefit plans
Fair value of plan assets		8,831,000
Level 1 | Equity - large-cap exchange-traded fund
Benefit plans
Fair value of plan assets	23,278,000	10,482,000
Level 1 | Equity - mid-cap exchange-traded funds
Benefit plans
Fair value of plan assets	3,379,000	2,544,000
Level 1 | Equity - small-cap exchange-traded funds
Benefit plans
Fair value of plan assets	1,645,000	1,176,000
Level 1 | Equity - international funds
Benefit plans
Fair value of plan assets	6,936,000	5,661,000
Level 2
Benefit plans
Fair value of plan assets	66,849,000	62,187,000
Level 2 | U.S. Treasury securities
Benefit plans
Fair value of plan assets	4,053,000	4,450,000
Level 2 | Government agency debt securities
Benefit plans
Fair value of plan assets	3,504,000	3,565,000
Level 2 | Fixed income - U.S. corporate securities
Benefit plans
Fair value of plan assets	58,808,000	53,721,000
Level 2 | Fixed income - municipal securities
Benefit plans
Fair value of plan assets	484,000	451,000
Level 3
Benefit plans
Fair value of plan assets	$ 0	$ 0