Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Available-for-Sale, Amortized Cost
Available-for-Sale Amortized Cost, Within One Year	$ 63.7
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,307.6
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	1,148.0
Available-for-Sale Amortized Cost, After Ten Years	3,574.0
Amortized Cost	6,093.3	$ 6,307.8
Available-for-Sale, Estimated Fair Value
Available-for-Sale Estimated Fair Value, Within One Year	62.7
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,213.5
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	1,044.3
Available-for-Sale Estimated Fair Value, After Ten Years	3,282.6
Estimated Fair Value	5,603.1	5,841.5
Held-to-Maturity, Amortized Cost
Held-to-Maturity Amortized Cost, Within One Year	2.5
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	430.7
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	557.3
Held-to-Maturity Amortized Cost, After Ten Years	1,808.7
Amortized Cost1	2,799.2	3,208.7
Held-to-Maturity, Estimated Fair Value
Held-to-Maturity Estimated Fair Value, Within One Year	2.5
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	404.0
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	487.0
Held-to-Maturity Estimated Fair Value, After Ten Years	1,546.9
Investment debt securities held-to-maturity	2,440.4	2,874.0
Securities Sold under Agreements to Repurchase [Abstract]
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	4,391.6	3,858.6
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure	3,902.1	$ 3,462.2
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Fair Value	1,541.6
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Amortized Cost Basis	$ 1,595.1