Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Available-for-Sale, Amortized Cost
Available-for-Sale Amortized Cost, Within One Year	$ 614.3
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,416.2
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	421.2
Available-for-Sale Amortized Cost, After Ten Years	495.0
Amortized Cost	2,946.7	$ 2,945.5
Available-for-Sale, Estimated Fair Value
Available-for-Sale Estimated Fair Value, Within One Year	608.0
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,422.1
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	523.5
Available-for-Sale Estimated Fair Value, After Ten Years	446.7
Available-for-Sale, Estimated Fair Value	3,000.3	2,960.0
Held-to-Maturity, Amortized Cost
Held-to-Maturity Amortized Cost, Within One Year	19.7
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	27.4
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	21.3
Held-to-Maturity Amortized Cost, After Ten Years	1.8
Amortized Cost	70.2	92.3
Held-to-Maturity, Estimated Fair Value
Held-to-Maturity Estimated Fair Value, Within One Year	19.8
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	27.8
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	23.2
Held-to-Maturity Estimated Fair Value, After Ten Years	1.8
Held-to-Maturity, Estimated Fair Value	$ 72.6	$ 94.5