Income Taxes - Additional Tax Disclosures (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 1987
Income taxes
Tax bad debt reserve			$ 3,000
Deferred tax liability not recorded on tax bad debt reserve balance	$ 632	$ 1,000
Reconciliation of the beginning and ending amount of unrecognized tax benefits
Balance at beginning of year	475	475
Additions based on tax positions related to the current year	0	0
Additions for tax positions of prior year	0	0
Reductions for tax positions of prior years	0	0
Settlements	0	0
Balance at end of year	475	475
Interest accrued during the period related to unrecognized tax benefits	5	$ 4
Federal
Income taxes
Net operating loss carryforwards	9,800
Federal | General Business Tax Credit Carryforward
Income taxes
Tax credit carryforward	1,500
Federal | Alternative Minimum Tax Credit Carryforward
Income taxes
Tax credit carryforward	228
California
Income taxes
Net operating loss carryforwards	$ 27,000