Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Available-for-Sale, Amortized Cost
Available-for-Sale Amortized Cost, Within One Year	$ 112.5
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,303.3
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	1,254.5
Available-for-Sale Amortized Cost, After Ten Years	2,541.0
Amortized Cost	5,211.3	$ 5,474.8
Available-for-Sale, Estimated Fair Value
Available-for-Sale Estimated Fair Value, Within One Year	111.3
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,240.8
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	1,180.6
Available-for-Sale Estimated Fair Value, After Ten Years	2,331.6
Estimated Fair Value	4,864.3	5,057.1
Held-to-Maturity, Amortized Cost
Held-to-Maturity Amortized Cost, Within One Year	100.1
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	465.8
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	443.6
Held-to-Maturity Amortized Cost, After Ten Years	1,630.8
Amortized Cost(1)	2,640.3	2,688.4
Held-to-Maturity, Estimated Fair Value
Held-to-Maturity Estimated Fair Value, Within One Year	99.8
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	443.5
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	395.2
Held-to-Maturity Estimated Fair Value, After Ten Years	1,422.7
Investment debt securities held-to-maturity	2,361.2	2,358.6
Securities Sold under Agreements to Repurchase [Abstract]
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	3,234.9	3,460.2
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure	2,945.8	$ 3,092.6
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Fair Value	1,043.2
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Amortized Cost Basis	$ 1,081.3