Revenue - Effects of Topic 606 on Financial Statements (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Assets
Accounts receivable, net	$ 145,182		$ 150,307	$ 145,385
Costs in excess of billings	22,214		16,532	16,532
Inventories	90,236		81,637	86,372
Total current assets	442,871		462,951	462,764
Total assets	967,143		991,572	991,385
Liabilities
Accrued expenses	53,254		75,380	75,467
Total current liabilities	145,917		170,946	171,033
Shareholders' equity
Retained earnings	283,538		274,836	274,562
Total shareholders' equity	541,955		531,993	531,719
Total liabilities and shareholders' equity	967,143		991,572	991,385
Net sales:	215,337	$ 206,605
Cost of sales	167,019	157,350
Gross profit	48,318	49,255
Provision for income taxes	2,807	2,053
Net income	8,352	$ 3,996
Calculated under Revenue Guidance in Effect before Topic 606
Assets
Accounts receivable, net	138,183			145,385
Costs in excess of billings				11,610
Inventories	96,371			86,372
Total current assets	442,007			462,764
Total assets	966,279			991,385
Liabilities
Accrued expenses	53,044			75,467
Total current liabilities	145,707			171,033
Shareholders' equity
Retained earnings	282,884			274,562
Total shareholders' equity	541,301			531,719
Total liabilities and shareholders' equity	966,279			$ 991,385
Net sales:	213,369
Cost of sales	165,580
Gross profit	47,789
Provision for income taxes	2,658
Net income	7,972
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Assets
Accounts receivable, net	6,999		4,922
Costs in excess of billings			4,922
Inventories	(6,135)		(4,735)
Total current assets	864		187
Total assets	864		187
Liabilities
Accrued expenses	210		(87)
Total current liabilities	210		(87)
Shareholders' equity
Retained earnings	654		274
Total shareholders' equity	654		274
Total liabilities and shareholders' equity	864		$ 187
Net sales:	1,968
Cost of sales	1,439
Gross profit	529
Provision for income taxes	149
Net income	$ 380