Transfers of financial assets and servicing assets - Key economic assumptions used (Details) - Residential Mortgage	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Assumption for Fair Value as of Balance Sheet Date of Assets or Liabilities that relate to Transferor's Continuing Involvement
Prepayment speed	6.80%	4.40%	6.90%	4.40%
Weighted average life	9 years 7 months 6 days	11 years 4 months 24 days	9 years 7 months 6 days	11 years 4 months 24 days
Discount rate (annual rate)	10.80%	11.00%	10.90%	11.10%