Allowance for loan losses - Troubled debt restructurings loan count by type of modification (Details) - loans	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Troubled Debt Restructuring
Reduction In Interest Rate	388	691	674	1,014
Extension of Maturity Date	63	77	127	131
Combination Interest Rate Reduction Maturity Date Extension	137	75	299	117
Other Modifications	57	182	123	355
Commercial multi-family
Troubled Debt Restructuring
Reduction In Interest Rate	0	0	0	0
Extension of Maturity Date	1	1	1	1
Combination Interest Rate Reduction Maturity Date Extension	0	0	0	0
Other Modifications	0	0	0	0
CRE non-owner occupied
Troubled Debt Restructuring
Reduction In Interest Rate	0	0	0	2
Extension of Maturity Date	2	6	3	11
Combination Interest Rate Reduction Maturity Date Extension	0	0	0	0
Other Modifications	0	0	0	0
CRE owner occupied
Troubled Debt Restructuring
Reduction In Interest Rate	1	3	1	3
Extension of Maturity Date	5	23	15	42
Combination Interest Rate Reduction Maturity Date Extension	0	0	0	0
Other Modifications	0	0	0	0
Commercial and industrial
Troubled Debt Restructuring
Reduction In Interest Rate	0	1	0	4
Extension of Maturity Date	18	31	34	50
Combination Interest Rate Reduction Maturity Date Extension	0	0	0	0
Other Modifications	0	0	0	0
Construction
Troubled Debt Restructuring
Reduction In Interest Rate		0		1
Extension of Maturity Date		0		0
Combination Interest Rate Reduction Maturity Date Extension		0		0
Other Modifications		0		0
Mortgages
Troubled Debt Restructuring
Reduction In Interest Rate	25	26	31	45
Extension of Maturity Date	28	6	55	10
Combination Interest Rate Reduction Maturity Date Extension	130	67	287	103
Other Modifications	2	22	2	45
Leasing
Troubled Debt Restructuring
Reduction In Interest Rate	0	0	0	0
Extension of Maturity Date	0	0	0	0
Combination Interest Rate Reduction Maturity Date Extension	1	1	1	1
Other Modifications	0	0	0	0
Credit cards
Troubled Debt Restructuring
Reduction In Interest Rate	161	180	283	311
Extension of Maturity Date	0	0	0	0
Combination Interest Rate Reduction Maturity Date Extension	0	3	1	3
Other Modifications	54	160	120	310
Home equity lines of credit
Troubled Debt Restructuring
Reduction In Interest Rate	0	0	0	0
Extension of Maturity Date	6	7	12	12
Combination Interest Rate Reduction Maturity Date Extension	5	3	9	7
Other Modifications	0	0	0	0
Personal Loan
Troubled Debt Restructuring
Reduction In Interest Rate	192	468	344	628
Extension of Maturity Date	1	1	3	3
Combination Interest Rate Reduction Maturity Date Extension	0	0	0	0
Other Modifications	1	0	1	0
Auto Loan
Troubled Debt Restructuring
Reduction In Interest Rate	0	0	0	0
Extension of Maturity Date	2	2	4	2
Combination Interest Rate Reduction Maturity Date Extension	1	1	1	2
Other Modifications	0	0	0	0
Other
Troubled Debt Restructuring
Reduction In Interest Rate	9	13	15	20
Extension of Maturity Date	0	0	0	0
Combination Interest Rate Reduction Maturity Date Extension	0	0	0	1
Other Modifications	0	0	0	0