Fair value measurements - Assets Measured on a Nonrecurring Basis (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Carrying or notional amount
Fair value measurements on a recurring basis
Loans, net	$ 4,695,508,000	$ 4,628,381,000
Estimated fair value
Fair value measurements on a recurring basis
Loans, net	4,780,250,000	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	7,380,000	11,254,000
Estimated fair value | Level 3
Fair value measurements on a recurring basis
Loans, net	4,772,870,000	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	545,000	2,621,000
Fair value measurements on a nonrecurring basis | Estimated fair value
Fair value measurements on a recurring basis
Loans, net	$ 545,000	$ 2,621,000