CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
Net income	$ 293,814	$ 190,993	$ 406,328
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred income taxes	60,420	41,875	(196,054)
Depreciation and amortization	13,840	13,874	13,872
Stock-based compensation	1,698	2,056	3,233
Amortization of debt discount and issuance costs	8,397	7,799	7,748
Loss on write-off of property and equipment	0	104	0
Loss on extinguishment of debt	0	216	5,745
Amortization of discount on short-term investments	(343)	(2,229)	(256)
Amortization of lease guarantee	(135)	(325)	(325)
Changes in fair values of equity investments	(50,277)	0	0
Other non-cash items	21	0	0
Changes in operating assets and liabilities:
Receivables from collaborative arrangements	(14,504)	3,859	(12,746)
Prepaid expenses and other current assets	(678)	(113)	(95)
Other assets	0	27	0
Accounts payable	56	(1)	(590)
Accrued personnel-related expenses and other accrued liabilities	804	(439)	(1,677)
Accrued interest payable	0	(112)	(1,656)
Other long-term liabilities	0	(126)	4
Net cash provided by operating activities	313,113	257,458	223,531
Cash flows from investing activities
Maturities of marketable securities	86,000	213,924	75,375
Purchases of marketable securities	(12,943)	(231,915)	(71,856)
Purchases of equity and other long term investments	(387,981)	0	0
Purchases of property and equipment	(13)	(12)	0
Net cash provided by (used in) investing activities	(314,937)	(18,003)	3,519
Cash flows from financing activities
Repurchase of shares to satisfy tax withholding	(92)	(89)	(3,073)
Payments of principal on senior secured term loans	0	(13,750)	(230,000)
Payments of cash dividends to stockholders	0	(11)	(80)
Proceeds from issuances of common stock, net	436	627	1,139
Net proceeds from the issuance of variable interest entity's equity	345	0	0
Distributions to noncontrolling interest	(30,474)	(10,553)	(5,955)
Net cash used in financing activities	(29,785)	(23,776)	(237,969)
Net increase (decrease) in cash and cash equivalents	(31,609)	215,679	(10,919)
Cash and cash equivalents at beginning of period	278,096	62,417	73,336
Cash and cash equivalents at end of period	246,487	278,096	62,417
Supplemental disclosure of cash flow information
Cash paid for interest	$ 9,933	$ 10,974	$ 17,861