Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Available-for-Sale, Amortized Cost
Available-for-Sale Amortized Cost, Within One Year	$ 88.7
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,326.9
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	1,166.4
Available-for-Sale Amortized Cost, After Ten Years	3,284.0
Amortized Cost	5,866.0	$ 6,307.8
Available-for-Sale, Estimated Fair Value
Available-for-Sale Estimated Fair Value, Within One Year	87.5
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,265.9
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	1,095.1
Available-for-Sale Estimated Fair Value, After Ten Years	3,085.8
Estimated Fair Value	5,534.3	5,841.5
Held-to-Maturity, Amortized Cost
Held-to-Maturity Amortized Cost, Within One Year	101.1
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	416.0
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	485.5
Held-to-Maturity Amortized Cost, After Ten Years	1,739.4
Amortized Cost1	2,742.0	3,208.7
Held-to-Maturity, Estimated Fair Value
Held-to-Maturity Estimated Fair Value, Within One Year	100.6
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	396.8
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	437.5
Held-to-Maturity Estimated Fair Value, After Ten Years	1,545.6
Investment debt securities held-to-maturity	2,480.5	2,874.0
Securities Sold under Agreements to Repurchase [Abstract]
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	3,159.8	3,858.6
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure	2,888.0	$ 3,462.2
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Fair Value	1,478.0
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Amortized Cost Basis	$ 1,514.6