Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
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	$ 40.1
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost	1,004.4
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost	1,373.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost	3,889.4
Debt Securities, Available-for-Sale, Excluding Accrued Interest	5,841.5	$ 6,946.1
Amortized Cost	6,307.8	7,573.5
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	39.4
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	938.7
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	1,255.4
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	3,608.0
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	309.1
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	346.6
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	654.0
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost	1,899.0
Amortized Cost1	3,208.7	3,453.7
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	307.5
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	329.4
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	579.1
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	1,658.0
Estimated Fair Value	2,874.0	3,052.2
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	1,552.2
Securities sold under agreements to repurchase	3,858.6	4,998.9
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure	$ 3,462.2	$ 4,432.0

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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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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