Fair value disclosures - Assets and liabilities measured at fair value on recurring basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	$ 52,429	$ 70,249
Fair Value, Inputs, Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	9,460	12,315
Fair Value, Inputs, Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	42,969	57,934
Money Market Funds
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	4,703	11,318
Money Market Funds | Fair Value, Inputs, Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	4,703	11,318
U.S. government securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	4,757	997
U.S. government securities | Fair Value, Inputs, Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	4,757	997
Money market accounts
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	23,959	46,955
Money market accounts | Fair Value, Inputs, Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	23,959	46,955
Corporate Bonds
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	7,811	5,676
Corporate Bonds | Fair Value, Inputs, Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	7,811	5,676
Government-sponsored enterprise obligations
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	4,292	3,284
Government-sponsored enterprise obligations | Fair Value, Inputs, Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	4,292	3,284
Commercial papers
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	6,285	1,398
Commercial papers | Fair Value, Inputs, Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	6,285	1,398
Sovereign government bonds
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	622	621
Sovereign government bonds | Fair Value, Inputs, Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	622	621
Mutual Funds Held in Rabbi Trust [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	599	435
Mutual Funds Held in Rabbi Trust [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	599	435
Penalty payment derivative
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Liabilities, fair value	389	389
Penalty payment derivative | Fair Value, Inputs, Level 3 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Liabilities, fair value	$ 389	$ 389