Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Available-for-Sale, Amortized Cost
Available-for-Sale Amortized Cost, Within One Year	$ 215.7
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,164.4
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	907.2
Available-for-Sale Amortized Cost, After Ten Years	3,682.7
Amortized Cost	5,970.0	$ 5,518.0
Available-for-Sale, Estimated Fair Value
Available-for-Sale Estimated Fair Value, Within One Year	214.0
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,112.7
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	835.1
Available-for-Sale Estimated Fair Value, After Ten Years	3,554.8
Estimated Fair Value	5,716.6	5,288.1
Held-to-Maturity, Amortized Cost
Held-to-Maturity Amortized Cost, Within One Year	46.0
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	640.0
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	400.5
Held-to-Maturity Amortized Cost, After Ten Years	1,207.4
Amortized Cost(1)	2,293.9	2,342.6
Held-to-Maturity, Estimated Fair Value
Held-to-Maturity Estimated Fair Value, Within One Year	45.9
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	605.0
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	364.1
Held-to-Maturity Estimated Fair Value, After Ten Years	1,067.7
Investment debt securities held-to-maturity	2,082.7	2,136.6
Securities Sold under Agreements to Repurchase [Abstract]
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	2,812.0	2,983.6
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure	2,604.3	$ 2,781.2
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Fair Value	604.9
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Amortized Cost Basis	$ 631.7