CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Operating activities:
Net income (loss)	$ (43,091)		$ 18,432	$ (145,340)	$ 99,304
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation				9,713	9,345
Amortization of right-of-use operating lease assets				1,356	1,433
Amortization of patents				1,526	1,139
Amortization of premium (discount) on investments, net				7,812	(7,064)
Amortization of debt issuance costs				1,884	1,448
Amortization of convertible senior notes discount				26,912	26,747
Stock-based compensation expense				135,077	111,564
Gain on investments	(835)		0	(10,722)	0
Non-cash losses related to patents, licensing and property, plant and equipment and investments				616	216
Provision for deferred income taxes				(3,475)	4,103
Changes in operating assets and liabilities:
Contracts receivable				24,057	(33,716)
Inventories				(1,468)	(6,606)
Other current and long-term assets				(5,647)	(23,377)
Income taxes				(23,674)	0
Accounts payable				(10,970)	(18,764)
Accrued compensation				(8,967)	(4,864)
Other current liabilities				13,195	3,909
Deferred contract revenue				(73,970)	(97,029)
Net cash provided by (used in) operating activities				(62,085)	67,788
Investing activities:
Purchases of short-term investments				(1,376,631)	(1,617,726)
Proceeds from sale of short-term investments				1,497,433	1,465,600
Purchases of property, plant and equipment				(29,971)	(23,143)
Acquisition of licenses and other assets, net				(4,203)	(4,196)
Net cash used in investing activities				86,628	(179,465)
Financing activities:
Proceeds from issuance of equity, net				29,450	112,137
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options				(12,960)	(18,841)
Repurchases and retirements of common stock		$ (90,600)		(90,548)	0	$ (34,400)
Payments of transaction costs for Akcea acquisition				(1,071)	0
Principal payments on debt obligations				0	(12,500)
Net cash provided by (used in) financing activities				(75,129)	80,796
Effect of exchange rates on cash				358	0
Net decrease in cash and cash equivalents				(50,228)	(30,881)
Cash and cash equivalents at beginning of period		$ 683,287		683,287	278,820	278,820
Cash and cash equivalents at end of period	$ 633,059		$ 247,939	633,059	247,939	$ 683,287
Supplemental disclosures of cash flow information:
Interest paid				3,700	5,474
Income taxes paid				23,532	9,037
Supplemental disclosures of non-cash investing and financing activities:
Right-of-use assets obtained in exchange for lease liabilities				0	14,178
Amounts accrued for capital and patent expenditures				6,576	3,251
Accrued amounts for Akcea acquisition transaction costs				$ 8,103	$ 0