Summary of significant accounting policies - Summary of Income Tax Expense Related to Investment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Tax Credit Carryforward [Line Items]
Net benefit to income tax expense	$ (50,797)	$ (109,393)	$ (123,695)
American Savings Bank (ASB)
Tax Credit Carryforward [Line Items]
Net benefit to income tax expense	(24,069)	(31,719)	(30,073)
American Savings Bank (ASB) | Variable Interest Entity, Not Primary Beneficiary
Tax Credit Carryforward [Line Items]
Amortization recognized in the provision for income taxes	(7,700)	(7,400)	(5,800)
Tax credits and other tax benefits recognized in the provision for income taxes	10,900	10,700	8,400
Net benefit to income tax expense	$ 3,200	$ 3,300	$ 2,600