Income Taxes (Components of Income Before Taxes and of Income Tax Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes
United States	$ 35,050	$ 21,634	$ 11,319
Foreign	961	2,312	1,456
Income before income taxes	36,011	23,946	12,775
Federal
Current	(58)	324	(208)
Deferred	(31,198)	8,807	4,871
Total	(31,256)	9,131	4,663
State
Current	284	14	13
Deferred	4,805	292	1,163
Total	5,089	306	1,176
Foreign
Current	20	440	476
Total	20	440	476
Total
Current	246	778	281
Deferred	(26,393)	9,099	6,034
Total	(26,147)	9,877	6,315
Reconciliation of the federal income tax expense at the statutory rate to the effective income tax expense
Statutory federal income tax expense	12,244	8,142	4,343
State taxes, net of federal benefit	3,360	202	776
Foreign tax paid	20	440	476
Change in federal tax rate	(43,643)
Tax consequences of the sale of engines to WMES	164		(306)
Uncertain tax positions		(40)	(195)
Permanent differences-nondeductible executive compensation	1,238	1,201	1,117
Permanent differences and other	470	(68)	104
Total	$ (26,147)	$ 9,877	$ 6,315
Reconciliation of the federal income tax expense
Statutory federal income tax expense (as a percent)	35.00%