Income Taxes (Components of Income Before Taxes and of Income Tax Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes
United States	$ 55,117	$ 35,050	$ 21,634
Foreign	1,157	961	2,312
Income before income taxes	56,274	36,011	23,946
Federal
Current		(58)	324
Deferred	12,871	(31,198)	8,807
Total	12,871	(31,256)	9,131
State
Current	364	284	14
Deferred	(814)	4,805	292
Total	(450)	5,089	306
Foreign
Current	622	20	440
Total	622	20	440
Total
Current	986	246	778
Deferred	12,057	(26,393)	9,099
Total	13,043	(26,147)	9,877
Reconciliation of the federal income tax expense at the statutory rate to the effective income tax expense
Statutory federal income tax expense	11,818	12,244	8,142
State taxes, net of federal benefit	(526)	3,360	202
Foreign tax paid	622	20	440
Change in federal tax rate		(43,643)
Tax consequences of the sale of engines to WMES		164
Uncertain tax positions			(40)
Permanent differences-nondeductible executive compensation	1,144	1,238	1,201
Permanent differences and other	(15)	470	(68)
Total	$ 13,043	$ (26,147)	$ 9,877
Reconciliation of the federal income tax expense
Statutory federal income tax expense (as a percent)	21.00%	35.00%