Fair value measurements - Assets Measured on a Nonrecurring Basis (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Estimated fair value
Fair value measurements on a recurring basis
Loans, net	$ 5,306,098,000	$ 5,157,537,000
Estimated fair value | Level 1
Fair value measurements on a recurring basis
Loans, net	0	0
Estimated fair value | Level 2
Fair value measurements on a recurring basis
Loans, net	18,155,000	12,295,000
Estimated fair value | Level 3
Fair value measurements on a recurring basis
Loans, net	5,287,943,000	5,145,242,000
Fair value measurements on a nonrecurring basis | American Savings Bank (ASB)
Fair value measurements on a recurring basis
Adjustments to fair value of loans held for sale	0		$ 0
Fair value measurements on a nonrecurring basis | Level 1
Fair value measurements on a recurring basis
Loans, net	0	0
Fair value measurements on a nonrecurring basis | Level 2
Fair value measurements on a recurring basis
Loans, net	0	0
Fair value measurements on a nonrecurring basis | Level 3
Fair value measurements on a recurring basis
Loans, net	0	25,000
Fair value measurements on a nonrecurring basis | Estimated fair value
Fair value measurements on a recurring basis
Loans, net	$ 0	$ 25,000