Revenue - Effects of Topic 606 on Financial Statements (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Accounts receivable, net	$ 180,875						$ 180,875		$ 150,307	$ 145,385
Costs in excess of billings	21,522						21,522		16,532	16,532
Inventories	97,486						97,486		81,637	86,372
Total current assets	532,723						532,723		462,951	462,764
Total assets	1,053,787						1,053,787		991,572	991,385
Liabilities
Accrued expenses	76,268						76,268		75,380	75,467
Total current liabilities	191,565						191,565		170,946	171,033
Shareholders' equity
Retained earnings	325,878						325,878		274,836	274,562
Total shareholders' equity	582,875	$ 560,896	$ 541,955	$ 505,431	$ 481,903	$ 466,305	582,875	$ 505,431	531,993	531,719	$ 460,880
Total liabilities and shareholders' equity	1,053,787						1,053,787		991,572	991,385
Net sales:	280,086			274,574			761,459	728,806
Cost of sales	209,807			205,839			572,359	548,991
Gross profit	70,279			68,735			189,100	179,815
Provision for income taxes	6,473			11,184			15,574	21,090
Net income	19,503	$ 22,837	$ 8,352	$ 20,619	$ 12,769	$ 3,996	50,692	$ 37,384
Calculated under Revenue Guidance in Effect before Topic 606
Assets
Accounts receivable, net	174,426						174,426			145,385
Costs in excess of billings										11,610
Inventories	102,662						102,662			86,372
Total current assets	531,450						531,450			462,764
Total assets	1,052,514						1,052,514			991,385
Liabilities
Accrued expenses	75,889						75,889			75,467
Total current liabilities	191,186						191,186			171,033
Shareholders' equity
Retained earnings	324,984						324,984			274,562
Total shareholders' equity	581,981						581,981			531,719
Total liabilities and shareholders' equity	1,052,514						1,052,514			$ 991,385
Net sales:	281,156						760,277
Cost of sales	210,878						572,039
Gross profit	70,278						188,238
Provision for income taxes	6,473						15,332
Net income	19,502						50,072
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Assets
Accounts receivable, net	6,449						6,449		4,922
Costs in excess of billings									4,922
Inventories	(5,176)						(5,176)		(4,735)
Total current assets	1,273						1,273		187
Total assets	1,273						1,273		187
Liabilities
Accrued expenses	379						379		(87)
Total current liabilities	379						379		(87)
Shareholders' equity
Retained earnings	894						894		274
Total shareholders' equity	894						894		274
Total liabilities and shareholders' equity	1,273						1,273		$ 187
Net sales:	(1,070)						1,182
Cost of sales	(1,071)						320
Gross profit	1						862
Provision for income taxes	0						242
Net income	$ 1						$ 620