Note 1 - Condensed Consolidated Financial Statements - Business Overview and Significant Accounting Policies Business Overview (Details) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)
Business Overview [Abstract]
Number of Reportable Business Segments | segment	2
Mortgage segment
Mortgage Insurance [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	$ 241,600.0
Direct Primary Mortgage Insurance RIF	60,900.0
Credit Risk Transfer Transactions | Mortgage segment
Mortgage Insurance [Abstract]
Direct Primary Mortgage Insurance RIF	$ 332.8	$ 275.2