Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Statement of Cash Flows [Abstract]
Net income	$ 36,826	$ 22,662	$ 111,320
Adjustments to reconcile net income to net cash provided by operating activities:
(Gain)/loss on sale of assets and other	(58)	286
Depreciation and amortization	9,734	9,758
Noncash lease expense	1,997	1,675
Deferred income taxes	1,430	1,371
Noncash compensation related to stock plans	277	4,105
Provision (benefit) of doubtful accounts	721	(3)
Changes in operating assets and liabilities:
Trade accounts receivable	(32,161)	(26,968)
Inventories	(5,962)	3,080
Trade accounts payable	11,018	1,915
Other current assets	(6,821)	323
Accrued liabilities and other current liabilities	(4,927)	(6,718)
Other noncurrent assets and liabilities	651	(1,838)
Net cash provided by operating activities	12,725	9,648
Cash flows from investing activities
Capital expenditures	(6,795)	(7,352)
Asset acquisitions, net of cash acquired	0	(3,492)
Proceeds from sale of property and equipment	561	38
Net cash used in investing activities	(6,234)	(10,806)
Cash flows from financing activities
Repurchase of common stock	(62,679)	(30,000)
Proceeds from line of credit	154,529	6,758
Repayments of line of credit and capital leases	(5,415)	(6,459)
Dividends paid	(10,169)	(9,901)
Cash paid on behalf of employees for shares withheld	(7,699)	(5,864)
Net cash provided by (used) in financing activities	68,567	(45,466)
Effect of exchange rate changes on cash and cash equivalents	(3,527)	(149)
Net increase (decrease) in cash and cash equivalents	71,531	(46,773)
Cash and cash equivalents at beginning of period	230,210	160,180	160,180
Cash and cash equivalents at end of period	301,741	113,407	$ 230,210
Noncash activity during the period
Noncash capital expenditures	289	284
Dividends declared but not paid	10,204	9,761
Contingent consideration for acquisition	0	309
Issuance of Company’s common stock for compensation	$ 322	$ 292