Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Available-for-Sale, Amortized Cost
Available-for-Sale Amortized Cost, Within One Year	$ 619.6
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,455.7
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	609.0
Available-for-Sale Amortized Cost, After Ten Years	544.7
Amortized Cost	3,229.0	$ 2,945.5
Available-for-Sale, Estimated Fair Value
Available-for-Sale Estimated Fair Value, Within One Year	722.3
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,381.5
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	698.6
Available-for-Sale Estimated Fair Value, After Ten Years	515.2
Available-for-Sale, Estimated Fair Value	3,317.6	2,960.0
Held-to-Maturity, Amortized Cost
Held-to-Maturity Amortized Cost, Within One Year	19.1
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	29.4
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	17.7
Held-to-Maturity Amortized Cost, After Ten Years	1.7
Amortized Cost	67.9	92.3
Held-to-Maturity, Estimated Fair Value
Held-to-Maturity Estimated Fair Value, Within One Year	19.2
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	30.5
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	19.7
Held-to-Maturity Estimated Fair Value, After Ten Years	1.7
Held-to-Maturity, Estimated Fair Value	$ 71.1	$ 94.5