Investment Securities - Narrative (Details) $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2021 USD ($) Investments	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) Investments
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	$ 1,685.5		$ 51.6
Investment securities in an unrealized loss position (securities) | Investments	260		181
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 0.0		$ 0.0
Municipal Debt Securities, at Carrying Value	$ 59.1
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Percent Debt Securities	70.40%
Debt Securities, Available-for-sale, Amortized Cost	$ 3,949.4		3,932.7
Debt Securities, Available-for-sale, Realized Gain	0.0	$ 0.0
Debt Securities, Held-to-maturity, Sold, Realized Gain (Loss)			0.0
Debt Securities, Available-for-sale, Realized Loss	0.0	$ 0.0
State, county and municipal securities
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	72.4		46.6
Debt Securities, Available-for-sale, Amortized Cost	449.2		$ 462.1
813940 Political Organizations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Municipal Debt Securities, at Carrying Value	41.6
Residential Mortgage Backed Securities
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	291.6
Callable Within One Year
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost of investment securities callable after one year but within five years	151.2
Fair value of investment securities callable after one year but within five years	149.1
Callable structured notes amortized costs	$ 0.0