Fair value measurement - Assets measured at fair value on nonrecurring basis (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other real estate	$ 123,922		$ 122,072
Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	3,151	$ 13,147
Other real estate	10,742	8,035
Other foreclosed assets	1,021	1,283
Total	14,914	22,465
Loans - write-down	(497)	(3,316)
Other real estate owned - write down	(1,368)	(1,889)
Other foreclosed assets - write down	(152)	(118)
Total assets measured - write down	(2,017)	(5,323)
Nonrecurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	0	0
Other real estate	0	0
Other foreclosed assets	0	0
Total	0	0
Nonrecurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	0	0
Other real estate	0	0
Other foreclosed assets	0	0
Total	0	0
Nonrecurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	3,151	13,147
Other real estate	10,742	8,035
Other foreclosed assets	1,021	1,283
Total	$ 14,914	$ 22,465