Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Investment securities:
Debt Securities, Held-to-maturity, Allowance for Credit Loss	$ 0.7	$ 0.8
Estimated fair value	$ 2,497.8	$ 2,874.0
Stockholders' equity:
Nonvoting, noncumulative preferred stock, shares authorized (in shares)	100,000	100,000
Nonvoting, noncumulative preferred stock, shares issued (in shares)	0	0
Nonvoting, noncumulative preferred stock, shares outstanding (in shares)	0	0
Preferred Stock, Shares Outstanding	0	0
Shares outstanding (in shares)	104,571,623	103,941,626
Common Stock, Shares, Issued	104,571,623	103,941,626
Common Stock, Shares Authorized	150,000,000
Common Stock, Par or Stated Value Per Share	$ 0.00001	$ 0.00001
Preferred Stock, No Par Value	$ 0	$ 0
Debt Securities, Available-for-Sale, Amortized Cost, after Allowance for Credit Loss	$ 6,266.0	$ 6,307.8
Estimated Fair Value	5,773.3	5,841.5
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest	$ 0.0	$ 0.0
Class A Common Stock
Stockholders' equity:
Shares outstanding (in shares)		103,941,626
Common Stock, Shares, Issued		103,941,626