Revenue - Effects of Topic 606 on Financial Statements (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Assets
Accounts receivable, net	$ 171,642		$ 171,642		$ 150,307	$ 145,385
Costs in excess of billings	20,163		20,163		16,532	16,532
Inventories	95,694		95,694		81,637	86,372
Total current assets	489,673		489,673		462,951	462,764
Total assets	1,009,920		1,009,920		991,572	991,385
Liabilities
Accrued expenses	65,174		65,174		75,380	75,467
Total current liabilities	169,648		169,648		170,946	171,033
Shareholders' equity
Retained earnings	306,375		306,375		274,836	274,562
Total shareholders' equity	560,896		560,896		531,993	531,719
Total liabilities and shareholders' equity	1,009,920		1,009,920		991,572	991,385
Net sales:	266,036	$ 247,627	481,373	$ 454,232
Cost of sales	195,533	185,802	362,552	343,152
Gross profit	70,503	61,825	118,821	111,080
Provision for income taxes	6,294	7,853	9,101	9,906
Net income	22,837	$ 12,769	31,189	$ 16,765
Calculated under Revenue Guidance in Effect before Topic 606
Assets
Accounts receivable, net	164,359		164,359			145,385
Costs in excess of billings						11,610
Inventories	101,915		101,915			86,372
Total current assets	488,611		488,611			462,764
Total assets	1,008,858		1,008,858			991,385
Liabilities
Accrued expenses	65,005		65,005			75,467
Total current liabilities	169,479		169,479			171,033
Shareholders' equity
Retained earnings	305,482		305,482			274,562
Total shareholders' equity	560,003		560,003			531,719
Total liabilities and shareholders' equity	1,008,858		1,008,858			$ 991,385
Net sales:	265,752		479,121
Cost of sales	195,581		361,161
Gross profit	70,171		117,960
Provision for income taxes	6,201		8,859
Net income	22,598		30,570
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Assets
Accounts receivable, net	7,283		7,283		4,922
Costs in excess of billings					4,922
Inventories	(6,221)		(6,221)		(4,735)
Total current assets	1,062		1,062		187
Total assets	1,062		1,062		187
Liabilities
Accrued expenses	169		169		(87)
Total current liabilities	169		169		(87)
Shareholders' equity
Retained earnings	893		893		274
Total shareholders' equity	893		893		274
Total liabilities and shareholders' equity	1,062		1,062		$ 187
Net sales:	284		2,252
Cost of sales	(48)		1,391
Gross profit	332		861
Provision for income taxes	93		242
Net income	$ 239		$ 619