Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Jun. 30, 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	814.4
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	1,655.7
Available-for-Sale Amortized Cost, After Ten Years	3,979.0
Amortized Cost	6,449.6	$ 7,573.5
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	761.3
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	1,472.1
Available-for-Sale Estimated Fair Value, After Ten Years	3,624.0
Estimated Fair Value	5,857.9	6,946.1
Held-to-Maturity, Amortized Cost
Held-to-Maturity Amortized Cost, Within One Year	308.1
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	346.6
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	654.3
Held-to-Maturity Amortized Cost, After Ten Years	2,009.4
Amortized Cost1	3,318.4	3,453.7
Held-to-Maturity, Estimated Fair Value
Held-to-Maturity Estimated Fair Value, Within One Year	301.7
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	322.9
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	567.9
Held-to-Maturity Estimated Fair Value, After Ten Years	1,732.2
Investment debt securities held-to-maturity	2,924.7	3,052.2
Securities Sold under Agreements to Repurchase [Abstract]
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	7,227.8	4,998.9
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure	$ 6,455.1	$ 4,432.0