Defined Benefit Pension Plans - Fair Values of Plan Investments by Asset Category (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	$ 10,206	$ 12,665	$ 11,890
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	1,050	926
Insurance Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	9,156	8,133
Securities (Assets) [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	0	3,606
Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	1,050	926
Fair Value, Inputs, Level 1 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	1,050	926
Fair Value, Inputs, Level 1 [Member] | Insurance Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	0	0
Fair Value, Inputs, Level 1 [Member] | Securities (Assets) [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	0	0
Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	9,156	11,739	11,103
Fair Value, Inputs, Level 3 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	0	0
Fair Value, Inputs, Level 3 [Member] | Insurance Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	9,156	8,133	8,020
Fair Value, Inputs, Level 3 [Member] | Securities (Assets) [Member]
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets	$ 0	$ 3,606	$ 3,083