Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Available-for-Sale, Amortized Cost
Available-for-Sale Amortized Cost, Within One Year	$ 621.4
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,454.9
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	236.8
Available-for-Sale Amortized Cost, After Ten Years	401.6
Amortized Cost	2,714.7	$ 2,305.2
Available-for-Sale, Estimated Fair Value
Available-for-Sale Estimated Fair Value, Within One Year	624.9
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,464.0
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	238.6
Available-for-Sale Estimated Fair Value, After Ten Years	404.7
Available-for-Sale, Estimated Fair Value	2,732.2	2,270.7
Held-to-Maturity, Amortized Cost
Held-to-Maturity Amortized Cost, Within One Year	38.3
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	31.4
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	21.9
Held-to-Maturity Amortized Cost, After Ten Years	3.3
Amortized Cost	94.9	406.8
Held-to-Maturity, Estimated Fair Value
Held-to-Maturity Estimated Fair Value, Within One Year	38.3
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	31.8
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	23.7
Held-to-Maturity Estimated Fair Value, After Ten Years	3.4
Held-to-Maturity, Estimated Fair Value	$ 97.2	$ 400.7