Investment Securities - Narrative (Details) $ in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2021 USD ($)	Sep. 30, 2021 USD ($) Investments	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) Investments
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity		$ 1,618.1		$ 51.6
Investment securities in an unrealized loss position (securities) | Investments		278		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		$ 56.9
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Percent Debt Securities		71.90%
Debt Securities, Available-for-sale, Amortized Cost		$ 4,408.8		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		69.7		46.6
Debt Securities, Available-for-sale, Amortized Cost		444.9		$ 462.1
813940 Political Organizations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Municipal Debt Securities, at Carrying Value		40.9
Residential Mortgage Backed Securities
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Available-for-sale, Amortized Cost		540.2
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		212.3
Fair value of investment securities callable after one year but within five years		209.9
Callable structured notes amortized costs		$ 0.0