Cash and Cash Equivalents and Securities - Debt Securities Held-to-maturity Credit Quality (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Debt securities held-to-maturity	$ 3,863.4	$ 3,869.2
Investment Grade
Schedule of Investments [Line Items]
Debt securities held-to-maturity	3,858.0
Non-Investment Grade
Schedule of Investments [Line Items]
Debt securities held-to-maturity	5.4
State and municipal
Schedule of Investments [Line Items]
Debt securities held-to-maturity	2,574.9	2,503.9
State and municipal | Investment Grade
Schedule of Investments [Line Items]
Debt securities held-to-maturity	2,574.5
State and municipal | Non-Investment Grade
Schedule of Investments [Line Items]
Debt securities held-to-maturity	0.4
GSE mortgage-backed securities
Schedule of Investments [Line Items]
Debt securities held-to-maturity	1,190.8	1,271.4
GSE mortgage-backed securities | Investment Grade
Schedule of Investments [Line Items]
Debt securities held-to-maturity	1,190.8
GSE mortgage-backed securities | Non-Investment Grade
Schedule of Investments [Line Items]
Debt securities held-to-maturity	0.0
Corporate
Schedule of Investments [Line Items]
Debt securities held-to-maturity	96.2	92.4
Corporate | Investment Grade
Schedule of Investments [Line Items]
Debt securities held-to-maturity	91.2
Corporate | Non-Investment Grade
Schedule of Investments [Line Items]
Debt securities held-to-maturity	5.0
Other
Schedule of Investments [Line Items]
Debt securities held-to-maturity	1.5	$ 1.5
Other | Investment Grade
Schedule of Investments [Line Items]
Debt securities held-to-maturity	1.5
Other | Non-Investment Grade
Schedule of Investments [Line Items]
Debt securities held-to-maturity	$ 0.0