Reconciliation of Total Revenue and Income Before Income Tax to Operating Revenue and Operating Income (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2026	Mar. 31, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Segment Reporting
Total Revenue	$ 2,547.5		$ 3,378.4	$ 9,272.7	$ 9,831.4
Gain (Loss) on Investments	(853.0)		128.0	(863.2)	(96.5)
Strategic Actions included within net investment income	0.0		0.0	(16.3)	0.0
Strategic Actions	0.0		0.0	(30.7)	0.0
Operating Segments
Segment Reporting
Total Revenue	2,547.5		3,378.4	9,272.7	9,831.4
Gain (Loss) on Investments	(853.0)		128.0	(863.2)	(96.5)
Segment Adjusted Operating Revenue	3,400.5		3,250.4	10,152.2	9,927.9
Income (Loss) Before Income Tax	(549.4)		54.5	82.8	715.1
Segment Adjusted Operating Income (Loss)	303.6		(73.5)	976.7	811.6
2026 Fortitude Re Reinsurance Transaction | Operating Segments
Segment Reporting
Gain (Loss) on Investments	(833.5)			(833.5)
2025 Fortitude Re Reinsurance Transaction | Operating Segments
Segment Reporting
Gain (Loss) on Investments		$ (46.8)	137.6		(46.8)
Unrelated to Significant Transaction | Operating Segments
Segment Reporting
Gain (Loss) on Investments	$ (19.5)		$ (9.6)	$ (29.7)	$ (49.7)