Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Available-for-Sale, Amortized Cost
Available-for-Sale Amortized Cost, Within One Year	$ 1.6
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,036.0
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	1,387.9
Available-for-Sale Amortized Cost, After Ten Years	3,864.7
Amortized Cost	6,290.2	$ 7,573.5
Available-for-Sale, Estimated Fair Value
Available-for-Sale Estimated Fair Value, Within One Year	1.5
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	941.3
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	1,220.0
Available-for-Sale Estimated Fair Value, After Ten Years	3,464.9
Estimated Fair Value	5,627.7	6,946.1
Held-to-Maturity, Amortized Cost
Held-to-Maturity Amortized Cost, Within One Year	308.7
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	346.4
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	654.8
Held-to-Maturity Amortized Cost, After Ten Years	1,950.4
Amortized Cost1	3,260.3	3,453.7
Held-to-Maturity, Estimated Fair Value
Held-to-Maturity Estimated Fair Value, Within One Year	304.5
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	320.2
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	551.5
Held-to-Maturity Estimated Fair Value, After Ten Years	1,619.9
Investment debt securities held-to-maturity	2,796.1	3,052.2
Securities Sold under Agreements to Repurchase [Abstract]
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	7,066.8	4,998.9
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure	$ 6,169.7	$ 4,432.0