Summary of significant accounting policies - Summary of Income Tax Expense Related to Investment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Amounts in income taxes related to low-income housing tax credit investments
Net benefit to income tax expense	$ (40,910)	$ (51,637)	$ (50,797)
American Savings Bank (ASB)
Amounts in income taxes related to low-income housing tax credit investments
Net benefit to income tax expense	(11,688)	(23,061)	(24,069)
American Savings Bank (ASB) | Variable Interest Entity, Not Primary Beneficiary
Amounts in income taxes related to low-income housing tax credit investments
Amortization recognized in the provision for income taxes	(9,600)	(7,900)	(7,700)
Tax credits and other tax benefits recognized in the provision for income taxes	13,700	11,900	10,900
Net benefit to income tax expense	$ 4,100	$ 4,000	$ 3,200