Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Available-for-Sale, Amortized Cost
Available-for-Sale Amortized Cost, Within One Year	$ 652.4
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,446.5
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	627.9
Available-for-Sale Amortized Cost, After Ten Years	642.4
Amortized Cost	3,369.2	$ 2,945.5
Available-for-Sale, Estimated Fair Value
Available-for-Sale Estimated Fair Value, Within One Year	931.3
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,251.2
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	864.5
Available-for-Sale Estimated Fair Value, After Ten Years	406.5
Available-for-Sale, Estimated Fair Value	3,453.5	2,960.0
Held-to-Maturity, Amortized Cost
Held-to-Maturity Amortized Cost, Within One Year	8.9
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	30.5
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	14.1
Held-to-Maturity Amortized Cost, After Ten Years	1.5
Amortized Cost	55.0	92.3
Held-to-Maturity, Estimated Fair Value
Held-to-Maturity Estimated Fair Value, Within One Year	8.9
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	31.9
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	16.1
Held-to-Maturity Estimated Fair Value, After Ten Years	1.5
Held-to-Maturity, Estimated Fair Value	58.4	94.5
Securities Sold under Agreements to Repurchase [Abstract]
Securities Sold under Agreements to Repurchase	820.3	697.6
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	2,097.6	2,132.0
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure	$ 2,165.0	$ 2,144.9