Cash and cash equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Cash, Cash Equivalents, and Short-term Investments [Abstract]
Cash and cash equivalents	$ 117,787	$ 82,387	$ 109,090	$ 61,653