Other Comprehensive Income - Schedule of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Equity [Abstract]
Other Comprehensive Income (Loss), Tax	$ (22.8)	$ (22.8)	$ 15.6	$ 12.4
Other Comprehensive Income (Loss), Transfers From Available-For-Sale To Held-To-Maturity Securities, Net Of Tax				(4.4)
Other Comprehensive Income (Loss), Net of Tax	(67.6)		45.6
Before Tax Amount
Change in net unrealized loss during period		(113.7)	61.8	54.9
Reclassification adjustment for net gains included in net income		(1.1)	(0.3)	(0.1)
OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), Tax		5.1
Change in net actuarial gains			(0.5)	(0.8)
Total other comprehensive loss				48.0
Tax Expense (Benefit)
Change in net unrealized gains during period		(28.7)	15.8	14.1
Reclassification adjustment for net gains included in net income		(0.3)	(0.1)	0.0
Change in net actuarial gains			(0.1)	(0.1)
Total other comprehensive income (loss)				12.4
Net of Tax Amount
Change in net unrealized gains during period		(85.0)	46.0	40.8
Reclassification adjustment for net gains included in net income		(0.8)	(0.2)	(0.1)
Change in net actuarial gains			(0.4)	(0.7)
Other comprehensive (loss) income, net of tax		(67.6)	45.6	35.6
Other comprehensive (loss) income, before tax	$ (90.4)	(90.4)	61.2	48.0
Net change in unamortized gains on available-for-sale investment securities transferred into held-to-maturity				(6.0)
Other Comprehensive Income (Loss), Transfers From Available-For-Sale To Held-To-Maturity Securities, Tax				(1.6)
OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), before Tax		20.2	0.0	0.0
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, Net of Tax		15.1
Other Comprehensive Income (Loss), Derivative, Excluded Component, Increase (Decrease), Adjustments, Tax		1.1	0.0
Other Comprehensive Income (Loss), Derivative, Excluded Component, Increase (Decrease), after Adjustments, Tax		3.1	0.2
Other Comprehensive Income (Loss), Derivative, Excluded Component, Increase (Decrease), after Adjustments, before Tax, Noncontrolling Interest		4.2	0.2
Unrealized Gain (Loss) on Derivatives		3.0	(0.2)
Stockholders' Equity Attributable to Parent		1,986.6	1,959.8	$ 2,013.9	$ 1,693.9
Debt Securities, Held-to-maturity, Transfer, Unrealized Gain (Loss)		$ 15.0	$ 0.0