Securities Purchased Under Agreements to Resell and Securities Sold Under Agreements to Repurchase - Repurchase Agreements Accounted for as Secured Borrowings (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total borrowings, repurchase agreements	$ 567.2	$ 531.9
Allowance for credit losses, available-for-sale securities	567.2	531.9
U.S. Treasury and Agency Securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total borrowings, repurchase agreements	567.2	531.9
OVERNIGHT AND CONTINUOUS
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total borrowings, repurchase agreements	567.2	32.4
Allowance for credit losses, available-for-sale securities	567.2	32.4
OVERNIGHT AND CONTINUOUS | U.S. Treasury and Agency Securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total borrowings, repurchase agreements	567.2	32.4
UP TO 30 DAYS
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total borrowings, repurchase agreements	0.0	499.5
Allowance for credit losses, available-for-sale securities	0.0	499.5
UP TO 30 DAYS | U.S. Treasury and Agency Securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total borrowings, repurchase agreements	$ 0.0	$ 499.5