Condensed Consolidated Balance Sheets (Unaudited) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 132,846	$ 57,514
Short-term investments	258,567	251,759
Receivables from collaborators	12	57
Prepaid expenses and other current assets	12,025	5,747
Total current assets	403,450	315,077
Property and equipment, net	3,211	2,776
Long-term investments	64,960	83,030
Prepaid and other long-term assets	14,678	15,612
Total assets	486,299	416,495
Current liabilities:
Accounts payable	5,019	14,084
Accrued compensation and employee benefits	1,678	3,512
Accrued and other liabilities	24,761	13,966
Deferred revenue, current portion	9,335	9,569
Total current liabilities	40,793	41,131
Deferred revenue, long-term	24,892	27,016
Other long-term liabilities	1,639	546
Total liabilities	67,324	68,693
Stockholders’ equity:
Preferred stock, $0.001 par value, 5,000,000 shares authorized at March 31, 2015 and December 31, 2014; no shares issued and outstanding at March 31, 2015 and December 31, 2014
Common stock, $0.001 par value, 100,000,000 shares authorized at March 31, 2015 and December 31, 2014; 52,037,514 and 48,766,806 shares issued and outstanding at March 31, 2015 and December 31, 2014, respectively	52	49
Additional paid-in capital	888,703	770,789
Accumulated deficit	(469,710)	(422,797)
Accumulated other comprehensive loss	(70)	(239)
Total stockholders’ equity	418,975	347,802
Total liabilities and stockholders’ equity	$ 486,299	$ 416,495