Transfers of financial assets and servicing assets - Key economic assumptions used (Details) - Residential Mortgage	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Assumption for Fair Value as of Balance Sheet Date of Assets or Liabilities that relate to Transferor's Continuing Involvement
Prepayment speed	7.50%	4.40%	6.90%	4.40%
Weighted average life	9 years 3 months 18 days	11 years 4 months 24 days	9 years 7 months 6 days	11 years 4 months 24 days
Discount rate (annual rate)	11.00%	11.10%	11.00%	11.10%