Significant accounting policies (Details)	12 Months Ended
	Dec. 31, 2017
Minimum
Significant Accounting Policies [Line Items]
Stock plans - assumed forfeiture rate	5.00%
Maximum
Significant Accounting Policies [Line Items]
Stock plans - assumed forfeiture rate	6.80%
Premiums
Significant Accounting Policies [Line Items]
Assumed risk period, in months	24 months
Cash and cash equivalents
Significant Accounting Policies [Line Items]
Time period for recognition, as shown	1 month