Fair value measurements - Assets Measured on a Nonrecurring Basis (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Estimated fair value
Fair value measurements on a recurring basis
Loans, net	$ 4,672,666,000	$ 4,781,751,000
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	1,031,000	11,254,000
Estimated fair value | Level 3
Fair value measurements on a recurring basis
Loans, net	4,671,635,000	4,770,497,000
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	77,000	2,621,000
Fair value measurements on a nonrecurring basis | Estimated fair value
Fair value measurements on a recurring basis
Loans, net	$ 77,000	$ 2,621,000