Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Investment securities:
Debt Securities, Held-to-maturity, Allowance for Credit Loss	$ 0.7	$ 1.9
Estimated fair value	$ 2,924.7	$ 3,052.2
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)	105,020,683	104,442,023
Common Stock, Shares, Issued	105,020,683	104,442,023
Common Stock, Shares Authorized	150,000,000	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	$ 6,448.2	$ 7,573.5
Estimated Fair Value	5,857.9	6,946.1
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Excluding Accrued Interest	$ 1.4	$ (0.0)
Class A Common Stock
Stockholders' equity:
Shares outstanding (in shares)		104,442,023
Common Stock, Shares, Issued		104,442,023