Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Available-for-Sale, Amortized Cost
Available-for-Sale Amortized Cost, Within One Year	$ 329.4
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,054.6
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	859.3
Available-for-Sale Amortized Cost, After Ten Years	3,790.3
Amortized Cost	6,033.6	$ 5,518.0
Available-for-Sale, Estimated Fair Value
Available-for-Sale Estimated Fair Value, Within One Year	326.0
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,000.5
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	786.9
Available-for-Sale Estimated Fair Value, After Ten Years	3,656.4
Estimated Fair Value	5,769.8	5,288.1
Held-to-Maturity, Amortized Cost
Held-to-Maturity Amortized Cost, Within One Year	69.2
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	595.5
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	447.4
Held-to-Maturity Amortized Cost, After Ten Years	1,103.8
Amortized Cost(1)	2,215.9	2,342.6
Held-to-Maturity, Estimated Fair Value
Held-to-Maturity Estimated Fair Value, Within One Year	68.5
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	559.2
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	404.6
Held-to-Maturity Estimated Fair Value, After Ten Years	968.0
Investment debt securities held-to-maturity	2,000.3	2,136.6
Securities Sold under Agreements to Repurchase [Abstract]
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	3,066.6	2,983.6
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure	2,832.7	$ 2,781.2
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Fair Value	653.6
Investment Securities, Primarily Classified As Available-For-Sale, Callable, Within One Year, Amortized Cost Basis	$ 681.3