Basis of Presentation and Significant Accounting Policies - Condensed Statements of Operations, Pro Forma Amounts Under Previous Guidance (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales	$ 162,523	$ 177,580	$ 327,221	$ 338,986
Cost of sales	135,764	133,829	263,260	254,201
Selling, general and administrative expenses	30,570	31,053	60,817	61,704
Provision for income taxes	2,088	1,415	1,922	1,121
Net income	$ 22,659	$ 4,684	$ 22,072	$ 4,803
Earnings per share:
Basic (in dollars per share)	$ 1.61	$ 0.33	$ 1.57	$ 0.33
Diluted (in dollars per share)	$ 1.60	$ 0.32	$ 1.55	$ 0.33
Without Adoption of ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales	$ 156,133		$ 328,321
Cost of sales	129,745		264,001
Selling, general and administrative expenses	30,427		60,837
Provision for income taxes	2,040		1,895
Net income	$ 22,478		$ 22,437
Earnings per share:
Basic (in dollars per share)	$ 1.60		$ 1.59
Diluted (in dollars per share)	$ 1.59		$ 1.58
Effect of Change | ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales	$ 6,390		$ (1,100)
Cost of sales	6,019		(741)
Selling, general and administrative expenses	143		(20)
Provision for income taxes	48		27
Net income	$ 181		$ (365)
Earnings per share:
Basic (in dollars per share)	$ 0.01		$ (0.03)
Diluted (in dollars per share)	$ 0.01		$ (0.03)