Note 1 - Description of Business - Mortgage (Details) - Mortgage segment - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Business Overview [Abstract]
Private Mortgage Insurance Protects Lenders For Loans Made With Less Than This Maximum Down Payment Percentage	20.00%
Private Mortgage Insurance Protects Lenders For Refinancings Made to Home Buyers With Less Than This Maximum Equity-Ownership Percentage	20.00%
Direct Primary Mortgage Insurance in Force	$ 246,100.0	$ 240,600.0
Direct Primary Mortgage Insurance Risk In Force	60,700.0	60,900.0
Credit Risk Transfer Transactions
Business Overview [Abstract]
Direct Primary Mortgage Insurance Risk In Force	$ 392.0	$ 275.2