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Business and Basis of Presentation (Details) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2022 USD ($) Company	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Jun. 30, 2022 USD ($) Company	Jun. 30, 2021 USD ($)
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Number of holding companies with outstanding public debt | Company	2			2
Pretax income (loss)	$ (22.0)	$ 125.0		$ 71.0	$ 140.0
Total consolidated	(47.0)	98.0		19.0	109.0
Fair value gains (losses) on financial guaranty variable interest entities	10.0	8.0		16.0	13.0
Foreign exchange gains (losses) on remeasurement	(71.0)	$ 5.0		(101.0)	$ 5.0
Revision of Prior Period, Error Correction, Adjustment
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Fair value gains (losses) on financial guaranty variable interest entities	$ 9.0			$ 6.9
Municipal Assurance Corp
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Write off of licenses			$ 16.0

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