Summary of significant accounting policies - Goodwill (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Goodwill
Operating cash flows forecast period assumed in goodwill impairment analysis (in years)	5 years
Minimum period for which terminal value is included from the balance sheet date for goodwill impairment analysis (in years)	5 years
Minimum
Goodwill
Terminal growth rate assumed in goodwill impairment analysis	2.20%	1.20%
Discount rate, pre-tax, used in goodwill impairment analysis	9.90%	7.50%
Maximum
Goodwill
Terminal growth rate assumed in goodwill impairment analysis	6.50%	4.90%
Discount rate, pre-tax, used in goodwill impairment analysis	19.30%	16.70%