Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Available-for-Sale, Amortized Cost
Available-for-Sale Amortized Cost, Within One Year	$ 624.8
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,516.3
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	1,062.4
Available-for-Sale Amortized Cost, After Ten Years	745.9
Amortized Cost	3,949.4	$ 3,932.7
Available-for-Sale, Estimated Fair Value
Available-for-Sale Estimated Fair Value, Within One Year	723.9
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,626.9
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	935.5
Available-for-Sale Estimated Fair Value, After Ten Years	671.5
Available-for-Sale, Estimated Fair Value	3,957.8	4,008.7
Held-to-Maturity, Amortized Cost
Held-to-Maturity Amortized Cost, Within One Year	682.2
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	393.3
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	272.9
Held-to-Maturity Amortized Cost, After Ten Years	337.1
Amortized Cost	1,685.5	51.6
Held-to-Maturity, Estimated Fair Value
Held-to-Maturity Estimated Fair Value, Within One Year	688.1
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	398.2
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	276.0
Held-to-Maturity Estimated Fair Value, After Ten Years	339.6
Held-to-Maturity, Estimated Fair Value	1,701.9	55.0
Securities Sold under Agreements to Repurchase [Abstract]
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	2,425.7	2,323.0
Securities Loaned or Sold under Agreements to Repurchase, Fair Value Disclosure	$ 2,456.0	$ 2,383.6