Fair value measurements - Fair Value Measurements on a Nonrecurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Estimated fair value
Fair value measurements on a recurring and nonrecurring basis
Loans	$ 5,439,330	$ 5,157,537
Level 1 | Estimated fair value
Fair value measurements on a recurring and nonrecurring basis
Loans	0	0
Level 2 | Estimated fair value
Fair value measurements on a recurring and nonrecurring basis
Loans	28,354	12,295
Level 3 | Estimated fair value
Fair value measurements on a recurring and nonrecurring basis
Loans	5,410,976	5,145,242
Fair value measurements on a nonrecurring basis
Fair value measurements on a recurring and nonrecurring basis
Loans	387	25
Fair value measurements on a nonrecurring basis | Estimated fair value
Fair value measurements on a recurring and nonrecurring basis
Mortgage servicing rights	3,001
Fair value measurements on a nonrecurring basis | Level 1
Fair value measurements on a recurring and nonrecurring basis
Loans	0	0
Mortgage servicing rights	0
Fair value measurements on a nonrecurring basis | Level 2
Fair value measurements on a recurring and nonrecurring basis
Loans	0	0
Mortgage servicing rights	0
Fair value measurements on a nonrecurring basis | Level 3
Fair value measurements on a recurring and nonrecurring basis
Loans	387	$ 25
Mortgage servicing rights	$ 3,001