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Summary of significant accounting policies - Summary of Income Tax Expense Related to Investment (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Tax Credit Carryforward [Line Items]      
Net benefit to income tax expense $ (109,393) $ (123,695) $ (93,021)
American Savings Bank (ASB)      
Tax Credit Carryforward [Line Items]      
Net benefit to income tax expense (31,719) (30,073) (29,082)
American Savings Bank (ASB) | Variable Interest Entity, Not Primary Beneficiary      
Tax Credit Carryforward [Line Items]      
Amortization recognized in the provision for income taxes (7,400) (5,800) (5,400)
Tax credits and other tax benefits recognized in the provision for income taxes 10,700 8,400 8,000
Net benefit to income tax expense $ 3,300 $ 2,600 $ 2,600