Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Amortized Cost	$ 64.7
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost	1,338.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost	1,131.4
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost	2,940.1
Debt Securities, Available-for-Sale, Excluding Accrued Interest	5,057.1	$ 5,841.5
Amortized Cost	5,474.8	6,307.8
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	63.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	1,258.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	1,048.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	2,685.4
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	101.2
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	435.9
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	471.3
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost	1,680.0
Amortized Cost(1)	2,688.4	3,208.7
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	100.7
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	408.5
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	413.6
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	1,435.8
Estimated Fair Value	2,358.6	2,874.0
Available-for-sale Securities and Held-to-maturity Securities
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Fair Value	1,247.2
Securities sold under agreements to repurchase	3,460.2	3,858.6
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure	$ 3,092.6	$ 3,462.2

Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Fair Value

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Pledged Assets Separately Reported, Securities Pledged for Repurchase Agreements, at Cost

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