Reconciliation of Total Revenue and Income Before Income Tax to Operating Revenue and Operating Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025	Dec. 31, 2025
Segment Reporting
Total Revenue	$ 3,370.0	$ 3,361.4	$ 6,725.2	$ 6,453.0
Gain (Loss) on Investments	(5.2)	(17.7)	(10.2)	(224.5)
Strategic Actions included within net investment income	16.3	0.0	16.3	0.0
Strategic Actions	(30.7)	0.0	(30.7)	0.0
Operating Segments
Segment Reporting
Total Revenue	3,370.0	3,361.4	6,725.2	6,453.0
Gain (Loss) on Investments	(5.2)	(17.7)	(10.2)	(224.5)
Segment Adjusted Operating Revenue	3,391.5	3,379.1	6,751.7	6,677.5
Income Before Income Tax	329.5	417.0	632.2	660.6
Segment Adjusted Operating Income	365.4	434.7	673.1	885.1
2025 Fortitude Re Reinsurance Transaction | Operating Segments
Segment Reporting
Gain (Loss) on Investments	0.0	(8.5)	0.0	(184.4)	$ (46.8)
Unrelated to Significant Transaction | Operating Segments
Segment Reporting
Gain (Loss) on Investments	$ (5.2)	$ (9.2)	$ (10.2)	$ (40.1)