Fair value measurements - Fair Value Measurements on a Nonrecurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Estimated fair value
Fair value measurements on a recurring and nonrecurring basis
Loans	$ 5,228,524	$ 5,439,330
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	10,403	28,354
Level 3 | Estimated fair value
Fair value measurements on a recurring and nonrecurring basis
Loans	$ 5,218,121	5,410,976
Fair value measurements on a nonrecurring basis | Estimated fair value
Fair value measurements on a recurring and nonrecurring basis
Loans		387
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
Mortgage servicing rights		0
Fair value measurements on a nonrecurring basis | Level 2
Fair value measurements on a recurring and nonrecurring basis
Loans		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
Mortgage servicing rights		$ 3,001