Allowance for loan losses - Quantitative information by loan class for loans modified as T D R (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) loans	Sep. 30, 2018 USD ($) loans	Sep. 30, 2019 USD ($) loans	Sep. 30, 2018 USD ($) loans
Troubled Debt Restructuring
Loan count | loans	622	880	1,845	2,496
Pre-Modification Recorded Investment	$ 49,279	$ 38,627	$ 116,599	$ 182,496
Post Modification Recorded Investment	47,493	37,046	111,076	176,711
Increase (decrease) in the allowance for loan losses as a result of modification	$ 1,529	$ 4,163	$ 6,887	$ 31,479
Commercial multi-family
Troubled Debt Restructuring
Loan count | loans		1	1	2
Pre-Modification Recorded Investment		$ 810	$ 154	$ 1,377
Post Modification Recorded Investment		808	116	1,375
Increase (decrease) in the allowance for loan losses as a result of modification		$ 63	$ (5)	$ 106
CRE non-owner occupied
Troubled Debt Restructuring
Loan count | loans	3	4	6	17
Pre-Modification Recorded Investment	$ 16,822	$ 1,523	$ 19,389	$ 28,969
Post Modification Recorded Investment	16,822	1,521	19,379	28,908
Increase (decrease) in the allowance for loan losses as a result of modification	$ (86)	$ 100	$ 721	$ 6,854
CRE owner occupied
Troubled Debt Restructuring
Loan count | loans	4	13	20	58
Pre-Modification Recorded Investment	$ 2,966	$ 7,578	$ 6,378	$ 27,648
Post Modification Recorded Investment	2,959	7,525	6,110	26,433
Increase (decrease) in the allowance for loan losses as a result of modification	$ (48)	$ 160	$ 9	$ 1,143
Commercial and industrial
Troubled Debt Restructuring
Loan count | loans	9	27	43	81
Pre-Modification Recorded Investment	$ 1,436	$ 2,411	$ 9,749	$ 49,633
Post Modification Recorded Investment	1,384	2,388	10,219	48,882
Increase (decrease) in the allowance for loan losses as a result of modification	$ 103	$ 139	$ 888	$ 13,963
Construction
Troubled Debt Restructuring
Loan count | loans				1
Pre-Modification Recorded Investment				$ 4,210
Post Modification Recorded Investment				4,293
Increase (decrease) in the allowance for loan losses as a result of modification				$ 474
Mortgages
Troubled Debt Restructuring
Loan count | loans	255	116	630	319
Pre-Modification Recorded Investment	$ 23,583	$ 15,143	$ 64,533	$ 40,741
Post Modification Recorded Investment	21,907	13,507	58,818	36,442
Increase (decrease) in the allowance for loan losses as a result of modification	$ 746	$ 640	$ 2,104	$ 1,874
Leasing
Troubled Debt Restructuring
Loan count | loans	2	3	3	4
Pre-Modification Recorded Investment	$ 237	$ 75	$ 264	$ 98
Post Modification Recorded Investment	238	73	266	96
Increase (decrease) in the allowance for loan losses as a result of modification	$ 0	$ 23	$ 7	$ 30
Credit cards
Troubled Debt Restructuring
Loan count | loans	153	187	557	811
Pre-Modification Recorded Investment	$ 1,375	$ 1,693	$ 4,690	$ 8,097
Post Modification Recorded Investment	1,355	1,838	4,823	8,642
Increase (decrease) in the allowance for loan losses as a result of modification	$ 109	$ 234	$ 466	$ 1,086
Home equity lines of credit
Troubled Debt Restructuring
Loan count | loans	4	10	25	29
Pre-Modification Recorded Investment	$ 276	$ 913	$ 2,359	$ 2,638
Post Modification Recorded Investment	246	906	2,256	2,579
Increase (decrease) in the allowance for loan losses as a result of modification	$ 25	$ 66	$ 345	$ 440
Personal Loan
Troubled Debt Restructuring
Loan count | loans	184	512	532	1,143
Pre-Modification Recorded Investment	$ 2,546	$ 8,026	$ 8,836	$ 18,351
Post Modification Recorded Investment	2,544	8,025	8,839	18,346
Increase (decrease) in the allowance for loan losses as a result of modification	$ 677	$ 2,660	$ 2,312	$ 5,390
Auto Loan
Troubled Debt Restructuring
Loan count | loans	1	5	6	8
Pre-Modification Recorded Investment	$ 7	$ 63	$ 70	$ 139
Post Modification Recorded Investment	7	63	73	122
Increase (decrease) in the allowance for loan losses as a result of modification	$ 1	$ 11	$ 13	$ 21
Other
Troubled Debt Restructuring
Loan count | loans	7	2	22	23
Pre-Modification Recorded Investment	$ 31	$ 392	$ 177	$ 595
Post Modification Recorded Investment	31	392	177	593
Increase (decrease) in the allowance for loan losses as a result of modification	$ 2	$ 67	$ 27	$ 98