Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Available-for-Sale, Amortized Cost
Available-for-Sale Amortized Cost, Within One Year	$ 0.5
Available-for-Sale Amortized Cost, After One Year but Within Five Years	786.6
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	1,686.5
Available-for-Sale Amortized Cost, After Ten Years	4,111.2
Amortized Cost	6,584.8
Available-for-Sale, Estimated Fair Value
Available-for-Sale Estimated Fair Value, Within One Year	0.5
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	746.1
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	1,524.1
Available-for-Sale Estimated Fair Value, After Ten Years	3,778.6
Estimated Fair Value	6,049.3	$ 6,946.1
Held-to-Maturity, Amortized Cost
Held-to-Maturity Amortized Cost, Within One Year	307.2
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	301.1
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	704.1
Held-to-Maturity Amortized Cost, After Ten Years	2,064.5
Amortized Cost	3,376.9	3,453.7
Held-to-Maturity, Estimated Fair Value
Held-to-Maturity Estimated Fair Value, Within One Year	300.4
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	287.2
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	624.4
Held-to-Maturity Estimated Fair Value, After Ten Years	1,813.8
Investment debt securities held-to-maturity	3,025.8	3,052.2
Securities Sold under Agreements to Repurchase [Abstract]
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	3,607.3	4,998.9
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure	$ 3,228.5	$ 4,432.0