CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating activities:
Net income (loss)	$ (486,766)	$ 303,262	$ 214,985
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation	13,365	12,540	10,706
Amortization of right-of-use operating lease assets	1,731	1,542	0
Amortization of patents	2,064	1,912	1,822
Amortization of premium (discount) on investments, net	11,521	(7,485)	(1,013)
Amortization of debt issuance costs	2,578	1,942	1,810
Amortization of convertible senior notes discount	36,157	37,338	33,363
Stock-based compensation expense	230,117	146,574	131,312
Loss on early retirement of debt	0	21,865	0
(Gain) loss on investments	(16,540)	(192)	210
Deferred income taxes, including changes in valuation allowance	313,272	(7,096)	(290,516)
Non-cash losses related to patents	1,948	2,226	802
Changes in operating assets and liabilities:
Contracts receivable	(13,170)	(47,674)	47,595
Inventories	(1,261)	(5,411)	1,400
Other current and long-term assets	(9,975)	(44,659)	(29,348)
Long-term income tax receivable	0	8,418	(223)
Accounts payable	(2,755)	(16,343)	(655)
Income taxes	(31,279)	31,656	(710)
Accrued compensation	28,371	8,089	4,117
Accrued liabilities and deferred rent	32,424	16,406	(17,005)
Deferred contract revenue	(75,910)	(119,283)	494,254
Net cash provided by operating activities	35,892	345,627	602,906
Investing activities:
Purchases of short-term investments	(1,570,410)	(1,946,726)	(1,794,735)
Proceeds from the sale of short-term investments	1,885,935	1,951,734	882,824
Purchases of property, plant and equipment	(35,120)	(30,905)	(13,608)
Acquisition of licenses and other assets, net	(5,928)	(5,377)	(4,044)
Purchase of strategic investments	0	(10,000)	0
Net cash provided by (used in) investing activities	274,477	(41,274)	(929,563)
Financing activities:
Proceeds from equity, net	52,036	119,657	27,900
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	(13,411)	(19,242)	0
Proceeds from the issuance of 0.125 percent convertible senior notes	0	109,500	0
0.125 percent convertible senior notes issuance costs	0	(10,428)	0
Proceeds from issuance of warrants	0	56,110	0
Purchase of note hedges	0	(108,684)	0
Repurchases and retirements of common stock	(90,548)	(34,392)	0
Purchase of noncontrolling interest of Akcea Therapeutics, Inc., including cash payments for cancellation of Akcea Therapeutics, Inc. equity awards	(544,686)	0	0
Principal payments on line of credit	0	(12,500)	0
Proceeds from the issuance of common stock to Biogen	0	0	447,965
Net cash (used in) provided by financing activities	(596,609)	100,021	475,865
Effects of exchange rates on cash	617	93	(18)
Net (decrease) increase in cash and cash equivalents	(285,623)	404,467	149,190
Cash and cash equivalents at beginning of year	683,287	278,820	129,630
Cash and cash equivalents at end of year	397,664	683,287	278,820
Supplemental disclosures of cash flow information:
Interest paid	6,247	9,870	9,592
Income taxes paid	25,855	9,041	0
Supplemental disclosures of non-cash investing and financing activities:
Right-of-use assets obtained in exchange for lease liabilities	2,149	14,178	0
Amounts accrued for capital and patent expenditures	4,059	3,126	4,428
Purchases of property, plant and equipment included in long-term obligations	0	0	3,350
0.125 percent convertible senior notes principal issued related to our December 2019 debt exchange/issuance	0	439,326	0
1 percent convertible senior notes principal extinguished related to our December 2019 debt exchange	$ 0	$ 375,590	$ 0