Fair value disclosures - Assets and liabilities measured at fair value on recurring basis (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	$ 70,321	$ 70,249
Fair Value, Inputs, Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	13,611	12,315
Fair Value, Inputs, Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	56,710	57,934
Money market funds [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	11,860	11,318
Money market funds [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	11,860	11,318
U.S. government securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	1,751	997
U.S. government securities | Fair Value, Inputs, Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	1,751	997
Money market accounts
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	31,957	46,955
Money market accounts | Fair Value, Inputs, Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	31,957	46,955
Corporate bonds [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	8,202	5,676
Corporate bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	8,202	5,676
Government-sponsored enterprise obligations
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	6,298	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	6,298	3,284
Commercial papers
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	9,632	1,398
Commercial papers | Fair Value, Inputs, Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	9,632	1,398
Sovereign government bonds [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	621	621
Sovereign government bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	621	621
Mutual Funds Held in Rabbi Trust [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	561	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	$ 561	$ 435