Summary of significant accounting policies - Goodwill (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
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	1.20%	2.70%
Discount rate, pre-tax, used in goodwill impairment analysis	7.50%	8.30%
Maximum
Goodwill
Terminal growth rate assumed in goodwill impairment analysis	4.90%	7.40%
Discount rate, pre-tax, used in goodwill impairment analysis	16.70%	16.90%