CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities
Net income	$ 86,080	$ 86,946
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred income taxes	18,940	0
Depreciation and amortization	7,080	6,936
Stock-based compensation	1,079	3,621
Amortization of debt discount and issuance costs	3,820	4,028
Amortization of discount on short-term investments	(1,124)	(160)
Amortization of lease guarantee	(162)	(162)
Loss on extinguishment of debt	0	3,137
Changes in operating assets and liabilities:
Receivables from collaborative arrangements	15,723	(2)
Prepaid expenses and other current assets	390	178
Accounts payable	1,128	(543)
Accrued personnel-related expenses and other accrued liabilities	343	(1,539)
Accrued interest payable	(2)	(805)
Operating lease liability	(144)
Other long-term liabilities	0	4
Net cash provided by operating activities	133,151	101,639
Cash flows from investing activities
Maturities of marketable securities	57,875	54,875
Purchases of marketable securities	(111,935)	(19,284)
Net cash provided by (used in) investing activities	(54,060)	35,591
Cash flows from financing activities
Repurchase of shares to satisfy tax withholding	(74)	(2,840)
Payments of principal on senior secured term loans	0	(120,000)
Payments of cash dividends to stockholders	(11)	(55)
Proceeds from issuances of common stock, net	529	454
Distributions to noncontrolling interest	0	(899)
Net cash provided by (used in) financing activities	444	(123,340)
Net increase in cash and cash equivalents	79,535	13,890
Cash and cash equivalents at beginning of period	62,417	73,336
Cash and cash equivalents at end of period	141,952	87,226
Supplemental disclosure of cash flow information
Cash paid for interest	$ 5,461	$ 10,913