Income taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of the beginning and ending amounts of unrecognized tax benefits [Roll Forward]
Balance at beginning of year		$ 1,003	$ 1,125
Decreases for tax positions taken during prior years		(1,003)	(122)
Balance at end of year		$ 0	$ 1,003
Reduction in unrecognized tax benefits resulting from lapse of statute of limitations	$ 1,000