Pension Plans (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension plans
Fair values of pension assets	$ 13,134,000	$ 11,440,000	$ 10,299,000
The excess plan
Pension plans
Fair values of pension assets	$ 1,182,000	$ 995,000