Summary of Significant Accounting Policies - Income Taxes and Retirement Plan (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes
Federal corporate income tax rate			35.00%	35.00%	35.00%
Reduction in the value of net deferred tax asset	$ 6,640,000		$ 6,640,000
Incremental income tax expense for remeasurement of deferred tax asset	6,640,000
Unrecognized tax benefit	0		0	$ 0
Retirement Plan
Pension asset	$ 1,450,000		1,450,000	332,000
Net (loss) gain recognized in other comprehensive income			$ 157,000	$ (123,000)	$ (513,000)
Forecast
Income Taxes
Federal corporate income tax rate		21.00%