Fair value measurement - Assets measured at fair value on nonrecurring basis (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Fair Value
Other real estate	$ 76,828		$ 83,146
Nonrecurring
Fair Value
Loans	22,700	$ 96,740
Loans held-for-sale		4,070
Other real estate	6,540	19,475
Other foreclosed assets		444
Long-lived asset held-for-sale	2,728
Total assets measured at fair value on a nonrecurring basis	31,968	120,729
Loans - write-down	(4,775)	(20,495)
Loans held-for-sale - write-down		(2,038)
Other real estate owned - write down	(1,583)	(3,025)
Other foreclosed assets - write down		(701)
Long-lived asset held-for-sale - write down	(303)
Total assets measured - write down	(6,661)	(26,259)
Nonrecurring | Level 1
Fair Value
Loans	0	0
Loans held-for-sale		0
Other real estate	0	0
Other foreclosed assets		0
Long-lived asset held-for-sale	0
Total assets measured at fair value on a nonrecurring basis	0	0
Nonrecurring | Level 2
Fair Value
Loans	0	0
Loans held-for-sale		0
Other real estate	0	0
Other foreclosed assets		0
Long-lived asset held-for-sale	0
Total assets measured at fair value on a nonrecurring basis	0	0
Nonrecurring | Level 3
Fair Value
Loans	22,700	96,740
Loans held-for-sale		4,070
Other real estate	6,540	19,475
Other foreclosed assets		444
Long-lived asset held-for-sale	2,728
Total assets measured at fair value on a nonrecurring basis	$ 31,968	$ 120,729