Summary of significant accounting policies - Goodwill (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Goodwill
Operating cash flows forecast period assumed in goodwill impairment analysis (in years)	4 years
Minimum period for which terminal value is included from the balance sheet date for goodwill impairment analysis (in years)	4 years
Minimum
Goodwill
Terminal growth rate assumed in goodwill impairment analysis	2.00%	2.20%
Discount rate, pre-tax, used in goodwill impairment analysis	10.10%	9.90%
Maximum
Goodwill
Terminal growth rate assumed in goodwill impairment analysis	6.50%	6.50%
Discount rate, pre-tax, used in goodwill impairment analysis	20.10%	19.30%