Debt - Maturities (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Aggregate maturities of finance leases
January 2020 - December 2020	$ 2,316,000
January 2021 - December 2021	53,000
January 2022 - December 2022	36,000
January 2023 - December 2023	7,000
Finance lease obligations	$ 2,412,000	$ 5,125,000
Minimum
Aggregate maturities of finance leases
Interest rate on leases	4.65%
Maximum
Aggregate maturities of finance leases
Interest rate on leases	5.37%
Notes payable to commercial banks
Notes payable
Aggregate principal amount outstanding		$ 5,975,000
Interest rate (as a percent)		5.00%
Notes payable to finance companies for insurance
Notes payable
Aggregate principal amount outstanding	$ 1,746,000	$ 1,680,000
Interest rate (as a percent)		3.80%
Notes payable to finance companies for insurance | Minimum
Notes payable
Interest rate (as a percent)	4.05%
Notes payable to finance companies for insurance | Maximum
Notes payable
Interest rate (as a percent)	4.99%