Fair value measurement - Assets measured at fair value on nonrecurring basis (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Fair Value
Other real estate	$ 72,060		$ 83,146
Nonrecurring
Fair Value
Loans	2,384	$ 3,151
Loans held-for-sale	3,549
Other real estate	6,205	10,742
Other foreclosed assets	48	1,021
Long-lived asset held-for-sale	2,728
Total assets measured at fair value on a nonrecurring basis	14,914	14,914
Loans - write-down	(406)	(497)
Loans held-for-sale - write-down	(846)
Other real estate owned - write down	(1,073)	(1,368)
Other foreclosed assets - write down	(7)	(152)
Long-lived asset held-for-sale - write down	(303)
Total assets measured - write down	(2,635)	(2,017)
Nonrecurring | Level 1
Fair Value
Loans	0	0
Loans held-for-sale	0
Other real estate	0	0
Other foreclosed assets	0	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	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	2,384	3,151
Loans held-for-sale	3,549
Other real estate	6,205	10,742
Other foreclosed assets	48	1,021
Long-lived asset held-for-sale	2,728
Total assets measured at fair value on a nonrecurring basis	$ 14,914	$ 14,914