Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Available-for-Sale, Amortized Cost
Available-for-Sale Amortized Cost, Within One Year	$ 576.7
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,522.7
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	256.3
Available-for-Sale Amortized Cost, After Ten Years	240.8
Amortized Cost	2,596.5	$ 2,305.2
Available-for-Sale, Estimated Fair Value
Available-for-Sale Estimated Fair Value, Within One Year	579.4
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,530.4
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	258.2
Available-for-Sale Estimated Fair Value, After Ten Years	241.7
Available-for-Sale, Estimated Fair Value	2,609.7	2,270.7
Held-to-Maturity, Amortized Cost
Held-to-Maturity Amortized Cost, Within One Year	28.9
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	42.1
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	23.9
Held-to-Maturity Amortized Cost, After Ten Years	3.9
Amortized Cost	98.8	406.8
Held-to-Maturity, Estimated Fair Value
Held-to-Maturity Estimated Fair Value, Within One Year	28.9
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	42.3
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	25.6
Held-to-Maturity Estimated Fair Value, After Ten Years	4.0
Held-to-Maturity, Estimated Fair Value	$ 100.8	$ 400.7