CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash Flows from Operating Activities:
Net income	$ 228,410	$ 198,156
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	42,976	41,696
Benefit of deferred income taxes	3,426	8,638
Share-based compensation expense	13,232	12,352
Other	747	1,613
Changes in assets and liabilities:
Accounts receivable	(37,699)	(32,872)
Inventories	(22,911)	(16,825)
Other assets and liabilities	(45,822)	(55,781)
Accounts payable	(4,030)	3
Deferred revenue	(6,849)	(3,252)
Net cash provided by operating activities	171,480	153,728
Cash Flows from Investing Activities:
Purchases of property and equipment	(71,987)	(51,377)
Purchase of marketable securities	0	(87)
Proceeds from the sale and maturities of marketable securities	0	284,125
Acquisition of a business	(304)	0
Net cash (used) provided by investing activities	(72,291)	232,661
Cash Flows from Financing Activities:
Repayments on revolving credit facilities, net	(147,519)	(218,000)
Issuance of senior notes	100,000	0
Debt issuance costs	(142)	0
Payment of acquisition-related contingent consideration	(1,695)	(1,000)
Repurchases of common stock	(74,994)	(189,884)
Proceeds from exercises of stock options and employee stock purchase plans	19,653	21,905
Shares withheld for statutory tax withholding on restricted stock	(7,572)	(8,720)
Net cash used by financing activities	(112,269)	(395,699)
Net effect of changes in exchange rates on cash	131	(3,806)
Net decrease in cash and cash equivalents	(12,949)	(13,116)
Cash and cash equivalents at beginning of period	123,794	187,675
Cash and cash equivalents at end of period	110,845	174,559
Supplemental Cash Flow Information:
Unpaid property and equipment, reflected in accounts payable and accrued liabilities	$ 11,633	$ 9,419