Debt (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023	Jul. 01, 2023	Mar. 31, 2000
Debt Instrument [Line Items]
Repurchase agreements	$ 2,731.0	$ 2,833.0
Long-term debt	14,379.0	13,035.0
Huatai Group [Member]
Debt Instrument [Line Items]
Repurchase agreements			$ 1,269.0
Variable Interest Entity, Primary Beneficiary
Debt Instrument [Line Items]
Repurchase agreements	815.0	1,009.0
Variable Interest Entity, Primary Beneficiary | Huatai Group [Member]
Debt Instrument [Line Items]
Repurchase agreements	$ 815.0	1,000.0
Huatai Group [Member]
Debt Instrument [Line Items]
Repurchase agreements			$ 1,300.0
Huatai Supplementary Bonds Due December 2034
Debt Instrument [Line Items]
Long-term debt stated interest rate	2.90%
Senior Notes | INA Senior Notes Due December 2051 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 600.0
Long-term debt stated interest rate	2.85%
Long-term debt	$ 594.0	593.0
Senior Notes | INA Senior Notes Due December 2061 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 1,000.0
Long-term debt stated interest rate	3.05%
Long-term debt	$ 984.0	984.0
Trust Preferred Securities | Chubb INA Capital Securities Due 2030 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 309.0			$ 300.0
ACE Capital Trust II common securities purchased				$ 9.2
Long-term debt stated interest rate	9.70%			9.70%
Unsecured Debt | INA Senior Notes Due August 2029 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 100.0
Long-term debt stated interest rate	8.875%
Long-term debt	$ 100.0	$ 100.0