Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Available-for-Sale, Amortized Cost
Available-for-Sale Amortized Cost, Within One Year	$ 43.9
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,188.4
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	1,216.0
Available-for-Sale Amortized Cost, After Ten Years	3,817.7
Amortized Cost	6,266.0	$ 6,307.8
Available-for-Sale, Estimated Fair Value
Available-for-Sale Estimated Fair Value, Within One Year	43.1
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,104.4
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	1,103.8
Available-for-Sale Estimated Fair Value, After Ten Years	3,522.0
Estimated Fair Value	5,773.3	5,841.5
Held-to-Maturity, Amortized Cost
Held-to-Maturity Amortized Cost, Within One Year	3.0
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	369.5
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	624.8
Held-to-Maturity Amortized Cost, After Ten Years	1,856.2
Amortized Cost1	2,853.5	3,208.7
Held-to-Maturity, Estimated Fair Value
Held-to-Maturity Estimated Fair Value, Within One Year	3.0
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	349.9
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	546.2
Held-to-Maturity Estimated Fair Value, After Ten Years	1,598.7
Investment debt securities held-to-maturity	2,497.8	2,874.0
Securities Sold under Agreements to Repurchase [Abstract]
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	3,427.6	3,858.6
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure	3,009.9	$ 3,462.2
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Fair Value	1,577.2
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Amortized Cost Basis	$ 1,632.5