Bank segment - Other borrowings (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Offsetting Liabilities [Line Items]
Gross amount of recognized liabilities	$ 91.0	$ 65.0
Gross amount offset in the Balance Sheets	0.0	0.0
Securities sold under agreements to repurchase	91.0	65.0
Commercial account holders
Offsetting Liabilities [Line Items]
Securities sold under agreements to repurchase	91.0	65.0
Securities sold under agreements to repurchase collateral, financial instruments	111.0	92.0
Securities sold under agreements to repurchase, cash collateral pledged	0.0	0.0
American Savings Bank (ASB)
Offsetting Liabilities [Line Items]
FHLB advances	38.0
Securities sold under agreements to repurchase	$ 91.2	$ 65.0