Note 10 - Income Taxes - Income Tax (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
US Treasury Securities
Operating Loss Carryforwards [Line Items]
Debt Securities	$ 313.1	$ 210.9
Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Income Taxes Payable, Federal, Current	25.2	$ 17.5
State and Local NOL Carryforwards
Operating Loss Carryforwards [Line Items]
Valuation Allowance, Amount	$ 84.7

Amount, after allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), investment in debt security measured at amortized cost (held-to-maturity), and investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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Carrying value as of the balance sheet date of obligations incurred and payable for statutory income, sales, use, payroll, excise, real, property and other taxes.

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