Fair value disclosures (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	$ 70,254	$ 8,297
Fair Value, Inputs, Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	14,445	4,587
Fair Value, Inputs, Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	55,809	3,710
Money Market Funds [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	13,442	4,587
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	13,442	4,587
Money Market Accounts [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	46,952
Money Market Accounts [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	46,952
Corporate Bonds [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	5,839	2,010
Corporate Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	5,839	2,010
US Government-sponsored Enterprises Debt Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	2,019
US Government-sponsored Enterprises Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	2,019
U.S. government securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	1,003
U.S. government securities | Fair Value, Inputs, Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	1,003
Commercial Papers
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	999
Commercial Papers | Fair Value, Inputs, Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	999
Variable Rate Demand Notes [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value		1,700
Variable Rate Demand Notes [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value		1,700
Mutual Funds Held in Rabbi Trust [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value	411	424
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	$ 411	$ 424