CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities:
Net income (loss)	$ 303,262	$ 214,985	$ (10,783)
Adjustments to reconcile net income (loss) to net cash provided by operating activities
Depreciation	12,540	10,706	6,708
Amortization of right-of-use operating lease assets	1,542	0	0
Amortization of patents	1,912	1,822	1,641
Amortization of premium (discount) on investments, net	(7,485)	(1,013)	6,752
Amortization of debt issuance costs	1,942	1,810	1,616
Amortization of convertible senior notes discount	37,338	33,363	30,920
Amortization of long-term financing liability for leased facility	0	0	3,659
Stock-based compensation expense	146,574	131,312	85,975
Gain on investment in Regulus Therapeutics, Inc.	0	0	(374)
Loss on extinguishment of financing liability for leased facility	0	0	7,689
Loss on early retirement of debt	21,865	0	0
Deferred income taxes (including benefit from valuation allowance release)	(7,096)	(290,516)	0
Non-cash losses related to patents, licensing, property, plant and equipment and investments	2,034	1,012	3,302
Changes in operating assets and liabilities:
Contracts receivable	(47,674)	47,595	45,088
Inventories	(5,411)	1,400	(2,493)
Other current and long-term assets	(44,659)	(29,348)	(58,367)
Long-term income tax receivable	8,418	(223)	(9,114)
Accounts payable	(16,343)	(655)	1,784
Income taxes	31,656	(710)	435
Accrued compensation	8,089	4,117	965
Accrued liabilities and deferred rent	16,499	(17,023)	28,564
Deferred contract revenue	(119,283)	494,254	30,182
Net cash provided by operating activities	345,720	602,888	174,149
Investing activities:
Purchases of short-term investments	(1,946,726)	(1,794,735)	(877,810)
Proceeds from sale of short-term investments	1,951,734	882,824	557,369
Purchases of property, plant and equipment	(30,905)	(13,608)	(34,764)
Acquisition of licenses and other assets, net	(5,377)	(4,044)	(3,093)
Purchase of strategic investments	(10,000)	0	(2,500)
Proceeds from the sale of Regulus Therapeutics, Inc.	0	0	2,507
Net cash (used in) provided by investing activities	(41,274)	(929,563)	(358,291)
Financing activities:
Proceeds from equity, net	119,657	27,900	22,931
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	(19,242)	0	0
Proceeds from the issuance of 0.125 percent convertible senior notes	109,500	0	0
0.125 percent convertible senior notes issuance costs	(10,428)	0	0
Proceeds from issuance of warrants	56,110	0	0
Purchase of note hedges	(108,684)	0	0
Repurchases and retirements of common stock	(34,392)	0	0
Principal payments on debt and capital lease obligations	(12,500)	0	(3,599)
Proceeds from issuance of common stock in Akcea Therapeutics, Inc. from its initial public offering, net of underwriters' discount	0	0	110,438
Proceeds from building mortgage debt, net of issuance costs	0	0	59,750
Proceeds from the sale of Akcea Therapeutics, Inc. common stock to Novartis in a private placement	0	0	50,000
Offering costs paid	0	0	(2,037)
Payment to settle financing liability for leased facility	0	0	(80,133)
Net cash provided by financing activities	100,021	475,865	229,087
Net increase in cash and cash equivalents	404,467	149,190	44,945
Cash and cash equivalents at beginning of year	278,820	129,630	84,685
Cash and cash equivalents at end of year	683,287	278,820	129,630
Supplemental disclosures of cash flow information:
Interest paid	9,870	9,592	8,035
Income taxes paid	9,041	0	0
Supplemental disclosures of non-cash investing and financing activities:
Right-of-use assets obtained in exchange for lease liabilities	14,178	0	0
Amounts accrued for capital and patent expenditures	3,126	4,428	1,983
Purchases of property, plant and equipment included in long-term obligations	0	3,350	0
0.125 percent convertible senior notes principal issued related to our December 2019 debt exchange/issuance	439,326	0	0
1 percent convertible senior notes principal extinguished related to our December 2019 debt exchange	375,590	0	0
Biogen [Member]
Financing activities:
Proceeds from the issuance of common stock	0	447,965	0
Novartis [Member]
Financing activities:
Proceeds from the issuance of common stock	$ 0	$ 0	$ 71,737