Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Available-for-Sale, Amortized Cost
Available-for-Sale Amortized Cost, Within One Year	$ 57.1
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,049.0
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	1,918.3
Available-for-Sale Amortized Cost, After Ten Years	4,423.8
Amortized Cost	7,448.2	$ 4,859.3
Available-for-Sale, Estimated Fair Value
Available-for-Sale Estimated Fair Value, Within One Year	56.4
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	998.0
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	1,710.2
Available-for-Sale Estimated Fair Value, After Ten Years	4,018.8
Available-for-Sale, Estimated Fair Value	6,783.4	4,820.5
Held-to-Maturity, Amortized Cost
Held-to-Maturity Amortized Cost, Within One Year	5.1
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	546.0
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	797.1
Held-to-Maturity Amortized Cost, After Ten Years	2,139.4
Amortized Cost	3,487.6	1,687.6
Held-to-Maturity, Estimated Fair Value
Held-to-Maturity Estimated Fair Value, Within One Year	5.0
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	524.2
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	695.2
Held-to-Maturity Estimated Fair Value, After Ten Years	1,839.1
Held-to-Maturity, Estimated Fair Value	3,063.5	1,667.5
Securities Sold under Agreements to Repurchase [Abstract]
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	4,836.7	2,617.8
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure	$ 4,265.1	$ 2,610.8