Allowance for credit losses - Quantitative information by loan class for loans modified as T D R (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) loans	Sep. 30, 2019 USD ($) loans	Sep. 30, 2020 USD ($) loans	Sep. 30, 2019 USD ($) loans shares
Financing Receivable
Loan count | loans	547	622	1,685	1,845
Pre-Modification Recorded Investment	$ 72,968,000	$ 49,279,000	$ 129,771,000	$ 116,599,000
Post Modification Recorded Investment	68,099,000	47,493,000	116,910,000	111,076,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 8,466,000	$ 1,529,000	$ 12,314,000	$ 6,887,000
Construction
Financing Receivable
Loan count | loans	1		1
Pre-Modification Recorded Investment	$ 21,514,000		$ 21,514,000
Post Modification Recorded Investment	21,514,000		21,514,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 4,370,000		$ 4,370,000
Mortgages
Financing Receivable
Loan count | loans	348	255	609	630
Pre-Modification Recorded Investment	$ 40,932,000	$ 23,583,000	$ 75,695,000	$ 64,533,000
Post Modification Recorded Investment	36,036,000	21,907,000	62,930,000	58,818,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 3,784,000	$ 746,000	$ 6,629,000	$ 2,104,000
Commercial | Commercial multi-family
Financing Receivable
Loan count	1		2	1
Pre-Modification Recorded Investment	$ 140,000		$ 1,133,000	$ 154,000
Post Modification Recorded Investment	139,000		1,115,000	116,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 1,000		$ (18,000)	$ (5,000)
Commercial | CRE non-owner occupied
Financing Receivable
Loan count | loans	2	3	5	6
Pre-Modification Recorded Investment	$ 5,060,000	$ 16,822,000	$ 8,478,000	$ 19,389,000
Post Modification Recorded Investment	5,058,000	16,822,000	8,476,000	19,379,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 0	$ (86,000)	$ (748,000)	$ 721,000
Commercial | CRE owner occupied
Financing Receivable
Loan count | loans	7	4	26	20
Pre-Modification Recorded Investment	$ 1,964,000	$ 2,966,000	$ 8,463,000	$ 6,378,000
Post Modification Recorded Investment	1,960,000	2,959,000	8,436,000	6,110,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 4,000	$ (48,000)	$ 110,000	$ 9,000
Commercial | Commercial and Industrial
Financing Receivable
Loan count | loans	12	9	39	43
Pre-Modification Recorded Investment	$ 625,000	$ 1,436,000	$ 2,409,000	$ 9,749,000
Post Modification Recorded Investment	614,000	1,384,000	2,392,000	10,219,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 0	$ 103,000	$ 25,000	$ 888,000
Consumer | Credit cards
Financing Receivable
Loan count | loans	82	153	699	557
Pre-Modification Recorded Investment	$ 1,032,000	$ 1,375,000	$ 6,492,000	$ 4,690,000
Post Modification Recorded Investment	1,076,000	1,355,000	6,530,000	4,823,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 14,000	$ 109,000	$ 276,000	$ 466,000
Consumer | Home equity lines of credit
Financing Receivable
Loan count | loans		4	2	25
Pre-Modification Recorded Investment		$ 276,000	$ 369,000	$ 2,359,000
Post Modification Recorded Investment		246,000	298,000	2,256,000
Increase (decrease) in the allowance for loan losses as a result of modification		$ 25,000	$ 862,000	$ 345,000
Consumer | Personal loans
Financing Receivable
Loan count | loans	92	184	273	532
Pre-Modification Recorded Investment	$ 1,682,000	$ 2,546,000	$ 4,703,000	$ 8,836,000
Post Modification Recorded Investment	1,682,000	2,544,000	4,701,000	8,839,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 290,000	$ 677,000	$ 781,000	$ 2,312,000
Consumer | Auto Loan
Financing Receivable
Loan count | loans	1	1	12	6
Pre-Modification Recorded Investment	$ 18,000	$ 7,000	$ 164,000	$ 70,000
Post Modification Recorded Investment	19,000	7,000	166,000	73,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 3,000	$ 1,000	$ 15,000	$ 13,000
Consumer | Other
Financing Receivable
Loan count | loans	1	7	3	22
Pre-Modification Recorded Investment	$ 1,000	$ 31,000	$ 25,000	$ 177,000
Post Modification Recorded Investment	1,000	31,000	25,000	177,000
Increase (decrease) in the allowance for loan losses as a result of modification	$ 0	$ 2,000	$ 6,000	$ 27,000
Leasing
Financing Receivable
Loan count | loans		2	14	3
Pre-Modification Recorded Investment		$ 237,000	$ 326,000	$ 264,000
Post Modification Recorded Investment		238,000	327,000	266,000
Increase (decrease) in the allowance for loan losses as a result of modification		$ 0	$ 6,000	$ 7,000