New Accounting Standards and Accounting Policies - Schedule of Changes Impact to Financial Results (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended					9 Months Ended		12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Net sales		$ 544,770	$ 543,275	$ 556,082	$ 522,760			$ 2,166,887	$ 2,239,557
Gross profit	$ 85,134	$ 79,628	$ 81,457	$ 85,145	$ 78,498	$ 259,147	$ 245,100	$ 324,728	$ 318,927
Gross profit margin		14.60%	15.00%	15.30%	15.00%			15.00%	14.20%
Operating profit (loss)	10,554	$ (993)	$ 1,506	$ 5,404	$ 4,711	29,991	11,621	$ 10,628	$ 34,901
Income tax expense (benefit)	2,383	(18)	474	1,187	(976)	6,653	685	667	11,158
Net income (loss)	$ 5,971	$ (3,176)	$ (767)	$ 2,366	$ 4,172	$ 16,665	$ 5,771	$ 2,595	$ 17,660
Earnings (Loss) Per Share: Basic	$ 0.50	$ (0.27)	$ (0.06)	$ 0.19	$ 0.34	$ 1.40	$ 0.46	$ 0.21	$ 1.49
Earnings (Loss) Per Share: Diluted	$ 0.47	$ (0.27)	$ (0.06)	$ 0.18	$ 0.31	$ 1.35	$ 0.43	$ 0.20	$ 1.41
Accounts receivable	$ 451,485	$ 439,658	$ 427,230	$ 454,729	$ 366,919	$ 451,485	$ 427,230	$ 439,658	$ 368,596
Inventory	63,989	103,471	61,598	66,231	55,086	63,989	61,598	103,471	72,219
Total current assets	533,012	561,575	507,235	542,796	447,889	533,012	507,235	561,575	470,049
Total assets	704,428	740,252	679,761	711,102	612,332	704,428	679,761	740,252	630,804
Accounts payable	312,079	289,201	272,112	356,063	241,706	312,079	272,112	289,201	276,704
Total current liabilities	516,478	569,294	506,321	537,139	447,242	516,478	506,321	569,294	474,232
Deferred income tax liability	4,051	3,102	4,190	4,082	1,966	4,051	4,190	3,102	1,815
Total liabilities	557,530	612,626	549,191	569,940	474,344	557,530	549,191	612,626	501,837
Retained Earnings	47,913	31,248	34,424	35,285	32,825	47,913	34,424	31,248	28,747
Total stockholders' equity	146,898	127,626	130,570	141,162	137,988	146,898	130,570	127,626	128,967
Total liabilities and stockholders' equity	$ 704,428	740,252	679,761	711,102	612,332	$ 704,428	679,761	740,252	630,804
As Reported [Member]
Net sales		563,448	545,479	560,110	524,399			2,193,436	2,250,587
Gross profit		$ 80,852	$ 81,294	$ 85,371	$ 78,205			$ 325,722	$ 318,801
Gross profit margin		14.30%	14.90%	15.20%	14.90%			14.80%	14.20%
Operating profit (loss)		$ (41)	$ 1,385	$ 5,624	$ 4,473			$ 11,441	$ 34,791
Income tax expense (benefit)		353	427	1,273	(1,069)			984	11,115
Net income (loss)		$ (2,595)	$ (841)	$ 2,500	$ 4,027			$ 3,091	$ 17,593
Earnings (Loss) Per Share: Basic		$ (0.22)	$ (0.07)	$ 0.20	$ 0.33			$ 0.25	$ 1.49
Earnings (Loss) Per Share: Diluted		$ (0.22)	$ (0.07)	$ 0.19	$ 0.30			$ 0.24	$ 1.40
Accounts receivable		$ 439,658	$ 412,733	$ 442,460	$ 354,301		412,733	$ 439,658	$ 358,949
Inventory		103,471	74,871	77,439	66,417		74,871	103,471	80,872
Total current assets		561,575	506,011	541,735	446,602		506,011	561,575	469,055
Total assets		740,252	678,537	710,041	611,045		678,537	740,252	629,810
Accounts payable		289,201	271,841	355,834	241,470		271,841	289,201	276,524
Total current liabilities		569,294	506,050	536,910	447,006		506,050	569,294	474,052
Deferred income tax liability		3,102	3,819	3,758	1,556		3,819	3,102	1,498
Total liabilities		612,626	548,548	569,386	473,698		548,548	612,626	501,339
Retained Earnings		31,248	33,843	34,778	32,184		33,843	31,248	28,251
Total stockholders' equity		127,626	129,989	140,655	137,347		129,989	127,626	128,471
Total liabilities and stockholders' equity		740,252	678,537	710,041	611,045		678,537	740,252	629,810
New Revenue Recognition Standard Adjustment [Member]
Net sales		(18,678)	(2,204)	(4,028)	(1,639)			(26,549)	(11,030)
Gross profit		$ (1,224)	$ 163	$ (226)	$ 293			$ (994)	$ 126
Gross profit margin		27.00%	9.00%	7.00%	10.00%			14.00%	8.00%
Operating profit (loss)		$ (952)	$ 121	$ (220)	$ 238			$ (813)	$ 110
Income tax expense (benefit)		(371)	47	(86)	93			(317)	43
Net income (loss)		$ (581)	$ 74	$ (134)	$ 145			$ (496)	$ 67
Earnings (Loss) Per Share: Basic		$ (0.05)	$ 0.01	$ (0.01)	$ 0.01			$ (0.04)	$ 0.01
Earnings (Loss) Per Share: Diluted		$ (0.05)	$ 0.01	$ (0.01)	$ 0.01			$ (0.04)	$ 0.01
Accounts receivable			$ 14,497	$ 12,269	$ 12,618		14,497		$ 9,647
Inventory			(13,273)	(11,208)	(11,331)		(13,273)		(8,653)
Total current assets			1,224	1,061	1,287		1,224		994
Total assets			1,224	1,061	1,287		1,224		994
Accounts payable			271	229	236		271		180
Total current liabilities			271	229	236		271		180
Deferred income tax liability			371	324	410		371		317
Total liabilities			643	554	646		643		498
Retained Earnings			581	507	641		581		496
Total stockholders' equity			581	507	641		581		496
Total liabilities and stockholders' equity			$ 1,224	$ 1,061	$ 1,287		$ 1,224		$ 994