CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net income	$ 190,993	$ 406,328	$ 134,272
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred income taxes	41,875	(196,054)	0
Depreciation and amortization	13,874	13,872	13,982
Stock-based compensation	2,056	3,233	9,833
Amortization of debt discount and issuance costs	7,799	7,748	5,116
Loss on write-off of property and equipment	104	0	0
Loss on extinguishment of debt	216	5,745	7,256
Amortization of discount on short-term investments	(2,229)	(256)	(105)
Amortization of lease guarantee	(325)	(325)	(325)
Changes in operating assets and liabilities:
Receivables from collaborative arrangements	3,859	(12,746)	(23,693)
Prepaid expenses and other current assets	(113)	(95)	12
Other assets	27	0	0
Accounts payable	(1)	(590)	473
Accrued personnel-related expenses and other accrued liabilities	(439)	(1,677)	(81)
Accrued interest payable	(112)	(1,656)	(1,908)
Other long-term liabilities	(126)	4	16
Deferred revenue	0	0	(3,099)
Net cash provided by operating activities	257,458	223,531	141,749
Cash flows from investing activities
Maturities of marketable securities	213,924	75,375	44,387
Purchases of marketable securities	(231,915)	(71,856)	(67,623)
Sales of marketable securities	0	0	0
Purchases of property and equipment	(12)	0	0
Net cash provided by (used in) investing activities	(18,003)	3,519	(23,236)
Cash flows from financing activities
Repurchase of shares to satisfy tax withholding	(89)	(3,073)	(2,128)
Payments of principal on senior secured term loans	(13,750)	(230,000)	(6,250)
Payments of cash dividends to stockholders	(11)	(80)	(281)
Proceeds from issuances of common stock, net	627	1,139	435
Proceeds from issuance of convertible senior notes due 2025	0	0	192,500
Proceeds from senior secured term loans	0	0	250,000
Payments of debt issuance costs and debt discount	0	0	(12,803)
Payments of principal on non-recourse notes due 2029	0	0	(487,189)
Repurchase of common stock	0	0	(97,500)
Contributions from (distributions to) noncontrolling interest	(10,553)	(5,955)	23
Net cash used in financing activities	(23,776)	(237,969)	(163,193)
Net increase (decrease) in cash and cash equivalents	215,679	(10,919)	(44,680)
Cash and cash equivalents at beginning of period	62,417	73,336	118,016
Cash and cash equivalents at end of period	278,096	62,417	73,336
Supplemental disclosure of cash flow information
Cash paid for interest	$ 10,974	$ 17,861	$ 40,353