Fair value measurements (Tables)

3 Months Ended

Mar. 31, 2018

Fair value measurements

Assets and liabilities measured at fair value on a recurring basis

Assets and liabilities measured at fair value on a recurring basis as of March 31, 2018 and December 31, 2017 are as follows:

			Quoted Prices		Significant
			in Active		Other		Significant
			Markets for		Observable		Unobservable
			Identical Assets		Inputs		Inputs
Assets	Total		(Level 1)		(Level 2)		(Level 3)
March 31, 2018	(in thousands)
Money market funds included in cash and cash equivalents	$	139,269	$	139,269	$	—	$	—
Marketable securities:
U.S. Treasury notes		77,827		77,827		—		—
Equity securities		1,320		1,320		—		—
Total marketable securities	$	79,147	$	79,147	$	—	$	—
Warrants to purchase equity securities		702		—		702		—
Total	$	219,118	$	218,416	$	702	$	—
December 31, 2017
Money market funds included in cash and cash equivalents	$	30,469	$	30,469	$	—	$	—
Marketable securities:
U.S. Treasury notes		137,522		137,522		—		—
Equity securities		1,100		1,100		—		—
Total marketable securities	$	138,622	$	138,622	$	—	$	—
Warrants to purchase equity securities		569		—		569		—
Total	$	169,660	$	169,091	$	569	$	—

Summary of fair value assumptions

The assumptions utilized to value the warrants to purchase equity securities as of March 31, 2018 and December 31, 2017 are as follows:

	As of March 31,		As of December 31,
	2018		2017
Risk-free interest rate	2.4	%	2.0	%
Expected dividend yield	—	%	—	%
Expected term (in years)	3.4		3.7
Expected volatility	104.3	%	103.5	%