CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net income	$ 406,328	$ 134,272	$ 59,536
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred income taxes	(196,054)	0	0
Depreciation and amortization	13,872	13,982	13,954
Stock-based compensation	3,233	9,833	8,297
Amortization of debt discount and issuance costs	7,748	5,116	2,847
Loss (gain) on extinguishment of debt	5,745	7,256	(2,342)
Amortization of discount on short-term investments	(256)	(105)	(9)
Amortization of lease guarantee	(325)	(325)	(190)
Interest added to the principal balance of non-recourse notes due 2029	0	0	855
Changes in operating assets and liabilities:
Receivables from collaborative arrangements	(12,746)	(23,693)	(20,619)
Prepaid expenses and other current assets	(95)	12	48
Other assets	0	0	(19)
Accounts payable	(590)	473	(690)
Accrued personnel-related expenses and other accrued liabilities	(1,677)	(81)	276
Accrued interest payable	(1,656)	(1,908)	(83)
Other long-term liabilities	4	16	8
Deferred revenue	0	(3,099)	(885)
Net cash provided by operating activities	223,531	141,749	60,984
Cash flows from investing activities
Maturities of marketable securities	75,375	44,387	88,422
Purchases of marketable securities	(71,856)	(67,623)	(95,719)
Sales of marketable securities	0	0	2,995
Purchases of property and equipment	0	0	(278)
Net cash provided by (used in) investing activities	3,519	(23,236)	(4,580)
Cash flows from financing activities
Repurchase of shares to satisfy tax withholding	(3,073)	(2,128)	(1,079)
Payments of principal on senior secured term loans	(230,000)	(6,250)	0
Payments of cash dividends to stockholders	(80)	(281)	(960)
Proceeds from issuances of common stock, net	1,139	435	385
Proceeds from issuance of convertible senior notes due 2025	0	192,500	0
Proceeds from senior secured term loans	0	250,000	0
Payments of debt issuance costs and debt discount	0	(12,803)	0
Payments of principal on non-recourse notes due 2029	0	(487,189)	(6,828)
Repurchase of common stock	0	(97,500)	(78,094)
Repurchase of convertible subordinated notes due 2023	0	0	(11,570)
Proceeds from capped-call options	0	0	578
Contributions from (distributions to) noncontrolling interest	(5,955)	23	0
Net cash used in financing activities	(237,969)	(163,193)	(97,568)
Net decrease in cash and cash equivalents	(10,919)	(44,680)	(41,164)
Cash and cash equivalents at beginning of period	73,336	118,016	159,180
Cash and cash equivalents at end of period	62,417	73,336	118,016
Supplemental disclosure of cash flow information
Cash paid for interest	$ 17,861	$ 40,353	$ 48,797