Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Available-for-Sale, Amortized Cost
Available-for-Sale Amortized Cost, Within One Year	$ 926.6
Available-for-Sale Amortized Cost, After One Year but Within Five Years	2,724.4
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	1,139.8
Available-for-Sale Amortized Cost, After Ten Years	1,784.3
Amortized Cost	6,575.1	$ 4,859.3
Available-for-Sale, Estimated Fair Value
Available-for-Sale Estimated Fair Value, Within One Year	886.6
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	2,592.0
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	1,074.3
Available-for-Sale Estimated Fair Value, After Ten Years	1,730.9
Available-for-Sale, Estimated Fair Value	6,283.8	4,820.5
Held-to-Maturity, Amortized Cost
Held-to-Maturity Amortized Cost, Within One Year	377.1
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	1,334.1
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	1,026.2
Held-to-Maturity Amortized Cost, After Ten Years	482.9
Amortized Cost	3,220.3	1,687.6
Held-to-Maturity, Estimated Fair Value
Held-to-Maturity Estimated Fair Value, Within One Year	355.3
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	1,274.0
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	956.0
Held-to-Maturity Estimated Fair Value, After Ten Years	443.8
Held-to-Maturity, Estimated Fair Value	3,029.1	1,667.5
Securities Sold under Agreements to Repurchase [Abstract]
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	2,587.0	2,617.8
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure	$ 2,421.8	$ 2,610.8