Fair value measurements - Fair Value Measurements on a Nonrecurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Estimated fair value
Fair value measurements on a recurring and nonrecurring basis
Loans	$ 4,852,826	$ 4,749,525
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	13,333	4,639
Level 3 | Estimated fair value
Fair value measurements on a recurring and nonrecurring basis
Loans	4,839,493	4,744,886
Fair value measurements on a nonrecurring basis | Estimated fair value
Fair value measurements on a recurring and nonrecurring basis
Loans	2,767	178
Real estate acquired in settlement of loans	1,189	1,030
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	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	0
Fair value measurements on a nonrecurring basis | Level 3
Fair value measurements on a recurring and nonrecurring basis
Loans	2,767	178
Real estate acquired in settlement of loans	$ 1,189	$ 1,030