Bank segment - Other borrowings (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Offsetting Liabilities [Line Items]
Gross amount  of recognized  liabilities	$ 103.0	$ 90.0
Gross amount  offset in the  Balance Sheets	0.0	0.0
Securities sold under agreements to repurchase	103.0	90.0
Commercial account holders
Offsetting Liabilities [Line Items]
Securities sold under agreements to repurchase	103.0	90.0
Financial instruments	122.0	92.0
Cash collateral pledged	0.0	0.0
American Savings Bank (ASB)
Offsetting Liabilities [Line Items]
Securities sold under agreements to repurchase	$ 102.7	$ 89.7