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	$ 536,058	$ 433,551
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation expense	183,478	160,572
Depreciation expense	54,838	34,402
Write-downs of inventories to net realizable value	7,042	6,928
Deferred income taxes	87,358	3,516
Unrealized gain on equity securities	(100,078)	(149,376)
Other non-cash items, net	(1,036)	10,014
Changes in operating assets and liabilities:
Accounts receivable, net	(55,870)	(88,166)
Inventories	(25,174)	(9,366)
Prepaid expenses and other assets	(17,580)	33,408
Accounts payable	(28,074)	12,229
Accrued expenses	113,968	101,074
Other liabilities	33,603	25,574
Net cash provided by operating activities	788,533	574,360
Cash flows from investing activities:
Purchases of available-for-sale debt securities	(263,636)	(202,002)
Maturities of available-for-sale debt securities	228,707	171,028
Expenditures for property and equipment	(34,399)	(58,891)
Investment in equity securities	(20,000)	(21,500)
Net cash used in investing activities	(89,328)	(111,365)
Cash flows from financing activities:
Issuances of common stock under benefit plans	114,092	144,837
Repurchases of common stock	(155,849)	(115,033)
Payments on finance leases	(18,926)
Advance from collaborator	7,500	5,000
Repayments of advanced funding	(2,823)	(2,412)
Proceeds related to capital lease and construction financing lease obligations	1,002	9,566
Payments on capital lease and construction financing lease obligations		(14,061)
Other financing activities	0	(149)
Net cash (used in) provided by financing activities	(55,004)	27,748
Effect of changes in exchange rates on cash	(808)	(4,201)
Net increase in cash and cash equivalents	643,393	486,542
Cash, cash equivalents and restricted cash—beginning of period	2,658,253	1,667,526
Cash, cash equivalents and restricted cash—end of period	3,301,646	2,154,068
Supplemental disclosure of cash flow information:
Cash paid for interest	27,109	33,444
Cash paid for income taxes	10,902	7,069
Capitalization of costs related to construction financing lease obligation	0	5,176
Issuances of common stock from employee benefit plans receivable	539	13,634
Accrued share repurchase liability	$ 0	$ 4,001