Allowance for credit losses - Quantitative information by loan class for loans modified as T D R (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) loans	Jun. 30, 2019 USD ($) loans	Jun. 30, 2020 USD ($) loans	Jun. 30, 2019 USD ($) loans shares
Financing Receivable
Loan count | loans	672	645	1,138	1,223
Pre-Modification Recorded Investment	$ 24,755,000	$ 34,890,000	$ 56,803,000	$ 67,320,000
Post Modification Recorded Investment	19,198,000	32,889,000	48,811,000	63,583,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 1,198,000	$ 3,299,000	$ 3,848,000	$ 5,358,000
Mortgages
Financing Receivable
Loan count | loans	86	185	261	375
Pre-Modification Recorded Investment	$ 14,448,000	$ 20,218,000	$ 34,763,000	$ 40,950,000
Post Modification Recorded Investment	8,954,000	18,320,000	26,894,000	36,911,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 833,000	$ 687,000	$ 2,845,000	$ 1,358,000
Commercial | Commercial multi-family
Financing Receivable
Loan count	1	1	1	1
Pre-Modification Recorded Investment	$ 993,000	$ 154,000	$ 993,000	$ 154,000
Post Modification Recorded Investment	976,000	116,000	976,000	116,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ (19,000)	$ (5,000)	$ (19,000)	$ (5,000)
Commercial | CRE non-owner occupied
Financing Receivable
Loan count | loans	1	2	3	3
Pre-Modification Recorded Investment	$ 179,000	$ 2,253,000	$ 3,418,000	$ 2,567,000
Post Modification Recorded Investment	179,000	2,246,000	3,418,000	2,557,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 0	$ 789,000	$ (748,000)	$ 807,000
Commercial | CRE owner occupied
Financing Receivable
Loan count | loans	13	6	19	16
Pre-Modification Recorded Investment	$ 2,593,000	$ 1,393,000	$ 6,499,000	$ 3,412,000
Post Modification Recorded Investment	2,574,000	1,178,000	6,476,000	3,151,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 1,000	$ 38,000	$ 106,000	$ 57,000
Commercial | Commercial and Industrial
Financing Receivable
Loan count | loans	15	18	27	34
Pre-Modification Recorded Investment	$ 849,000	$ 4,370,000	$ 1,784,000	$ 8,313,000
Post Modification Recorded Investment	845,000	4,356,000	1,778,000	8,835,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 2,000	$ 471,000	$ 25,000	$ 785,000
Consumer | Credit cards
Financing Receivable
Loan count | loans	456	215	617	404
Pre-Modification Recorded Investment	$ 4,200,000	$ 1,772,000	$ 5,460,000	$ 3,315,000
Post Modification Recorded Investment	4,181,000	1,938,000	5,454,000	3,468,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 178,000	$ 186,000	$ 262,000	$ 357,000
Consumer | Home equity lines of credit
Financing Receivable
Loan count | loans	1	11	2	21
Pre-Modification Recorded Investment	$ 37,000	$ 1,389,000	$ 369,000	$ 2,083,000
Post Modification Recorded Investment	33,000	1,389,000	298,000	2,010,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 8,000	$ 265,000	$ 862,000	$ 320,000
Consumer | Personal loans
Financing Receivable
Loan count | loans	79	194	181	348
Pre-Modification Recorded Investment	$ 1,160,000	$ 3,143,000	$ 3,021,000	$ 6,290,000
Post Modification Recorded Investment	1,160,000	3,142,000	3,019,000	6,295,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 194,000	$ 830,000	$ 491,000	$ 1,635,000
Consumer | Auto Loan
Financing Receivable
Loan count | loans	8	3	11	5
Pre-Modification Recorded Investment	$ 60,000	$ 38,000	$ 146,000	$ 63,000
Post Modification Recorded Investment	60,000	43,000	147,000	66,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 0	$ 8,000	$ 12,000	$ 12,000
Consumer | Other
Financing Receivable
Loan count | loans	1	9	2	15
Pre-Modification Recorded Investment	$ 4,000	$ 133,000	$ 24,000	$ 146,000
Post Modification Recorded Investment	4,000	133,000	24,000	146,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 1,000	$ 23,000	$ 6,000	$ 25,000
Leasing
Financing Receivable
Loan count | loans	11	1	14	1
Pre-Modification Recorded Investment	$ 232,000	$ 27,000	$ 326,000	$ 27,000
Post Modification Recorded Investment	232,000	28,000	327,000	28,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 0	$ 7,000	$ 6,000	$ 7,000