Goodwill and Core Deposit Intangibles - Schedule of Finite-Lived Intangible Assets (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
2023	$ 15.6
Finite-lived Intangible Assets [Roll Forward]
Gross other intangible assets, at beginning of the period		$ 106.0	$ 106.0
Provisional amounts established through acquisition	71.9
Accumulated amortization	(68.3)	(64.7)
Net other intangible assets, end of period	109.6	41.3
Core Deposits
Finite-Lived Intangible Assets [Line Items]
2023	13.7
Finite-lived Intangible Assets [Roll Forward]
Gross other intangible assets, at beginning of the period		106.0	106.0
Provisional amounts established through acquisition	49.1
Accumulated amortization	(68.0)	(64.7)
Net other intangible assets, end of period	87.1	41.3
OCRI
Finite-Lived Intangible Assets [Line Items]
2023	1.9
Finite-lived Intangible Assets [Roll Forward]
Gross other intangible assets, at beginning of the period		0.0	$ 0.0
Provisional amounts established through acquisition	22.8
Accumulated amortization	(0.3)	0.0
Net other intangible assets, end of period	$ 22.5	$ 0.0