Fair value measurements - Fair Value Measurements on a Nonrecurring Basis (Details) - Fair value measurements on a nonrecurring basis - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair value measurements on a recurring and nonrecurring basis
Loans	$ 25	$ 77
Real estate acquired in settlement of loans		186
Level 1
Fair value measurements on a recurring and nonrecurring basis
Loans	0	0
Real estate acquired in settlement of loans		0
Level 2
Fair value measurements on a recurring and nonrecurring basis
Loans	0	0
Real estate acquired in settlement of loans		0
Level 3
Fair value measurements on a recurring and nonrecurring basis
Loans	$ 25	77
Real estate acquired in settlement of loans		$ 186