Fair value measurements - Narrative (Details)	9 Months Ended
	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)
Fair value measurements on a nonrecurring basis | American Savings Bank (ASB)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Adjustments to fair value of loans held for sale	$ 0	$ 0
Weighted average discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Mortgage revenue bonds, measurement input	0.0565