Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Available-for-Sale, Amortized Cost
Available-for-Sale Amortized Cost, Within One Year	$ 977.9
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,124.4
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	1,062.5
Available-for-Sale Amortized Cost, After Ten Years	846.2
Amortized Cost	4,011.0	$ 3,932.7
Available-for-Sale, Estimated Fair Value
Available-for-Sale Estimated Fair Value, Within One Year	948.3
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,335.7
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	1,038.5
Available-for-Sale Estimated Fair Value, After Ten Years	697.6
Available-for-Sale, Estimated Fair Value	4,020.1	4,008.7
Held-to-Maturity, Amortized Cost
Held-to-Maturity Amortized Cost, Within One Year	341.0
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	195.6
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	147.5
Held-to-Maturity Amortized Cost, After Ten Years	182.2
Amortized Cost	866.3	51.6
Held-to-Maturity, Estimated Fair Value
Held-to-Maturity Estimated Fair Value, Within One Year	335.7
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	194.0
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	146.9
Held-to-Maturity Estimated Fair Value, After Ten Years	179.0
Held-to-Maturity, Estimated Fair Value	855.6	55.0
Securities Sold under Agreements to Repurchase [Abstract]
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	2,257.0		$ 2,323.0
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure	$ 2,290.8	$ 2,383.6