Fair value measurements - Fair Value Measurements on a Nonrecurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Estimated fair value
Fair value measurements on a recurring and nonrecurring basis
Loans	$ 4,802,053	$ 4,781,751
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	1,809	11,254
Level 3 | Estimated fair value
Fair value measurements on a recurring and nonrecurring basis
Loans	4,800,244	4,770,497
Fair value measurements on a nonrecurring basis | Estimated fair value
Fair value measurements on a recurring and nonrecurring basis
Loans	77	2,621
Real estate acquired in settlement of loans	186
Fair value measurements on a nonrecurring basis | Level 1
Fair value measurements on a recurring and nonrecurring basis
Loans	0	0
Real estate acquired in settlement of loans	0
Fair value measurements on a nonrecurring basis | Level 2
Fair value measurements on a recurring and nonrecurring basis
Loans	0	0
Real estate acquired in settlement of loans	0
Fair value measurements on a nonrecurring basis | Level 3
Fair value measurements on a recurring and nonrecurring basis
Loans	77	$ 2,621
Real estate acquired in settlement of loans	$ 186