Fair Value of Financial Instruments - Fair value hierarchy (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair value, assets measured on a recurring basis
Marketable securities	$ 26,358	$ 45,236
Financial instruments, fair value	37,491	53,096
Level 1
Fair value, assets measured on a recurring basis
Financial instruments, fair value	10,134	6,362
Level 2
Fair value, assets measured on a recurring basis
Financial instruments, fair value	27,357	46,734
Level 3
Fair value, assets measured on a recurring basis
Financial instruments, fair value	0	0
Money market funds
Fair value, assets measured on a recurring basis
Cash equivalents	10,134	6,362
Money market funds | Level 1
Fair value, assets measured on a recurring basis
Cash equivalents	10,134	6,362
Money market funds | Level 2
Fair value, assets measured on a recurring basis
Cash equivalents	0	0
Money market funds | Level 3
Fair value, assets measured on a recurring basis
Cash equivalents	0	0
Corporate notes and bonds
Fair value, assets measured on a recurring basis
Cash equivalents	999	1,498
Marketable securities	13,801	26,278
Corporate notes and bonds | Level 1
Fair value, assets measured on a recurring basis
Cash equivalents	0	0
Marketable securities	0	0
Corporate notes and bonds | Level 2
Fair value, assets measured on a recurring basis
Cash equivalents	999	1,498
Marketable securities	13,801	26,278
Corporate notes and bonds | Level 3
Fair value, assets measured on a recurring basis
Cash equivalents	0	0
Marketable securities	0	0
U.S. government and agency securities
Fair value, assets measured on a recurring basis
Marketable securities	3,483	11,976
U.S. government and agency securities | Level 1
Fair value, assets measured on a recurring basis
Marketable securities	0	0
U.S. government and agency securities | Level 2
Fair value, assets measured on a recurring basis
Marketable securities	3,483	11,976
U.S. government and agency securities | Level 3
Fair value, assets measured on a recurring basis
Marketable securities	0	0
Commercial paper
Fair value, assets measured on a recurring basis
Marketable securities	9,074	6,982
Commercial paper | Level 1
Fair value, assets measured on a recurring basis
Marketable securities	0	0
Commercial paper | Level 2
Fair value, assets measured on a recurring basis
Marketable securities	9,074	6,982
Commercial paper | Level 3
Fair value, assets measured on a recurring basis
Marketable securities	$ 0	$ 0