Debt (Detail) - Senior Notes [Member] - USD ($) $ in Millions	Sep. 30, 2024	Jul. 31, 2024	Mar. 07, 2024
INA Senior Notes Due March 2034 [Domain]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 1,600	$ 600	$ 1,000
Debt Instrument, Interest Rate, Stated Percentage		5.00%	5.00%
INA Senior Notes Due August 2029 [Domain]
Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 700
Debt Instrument, Interest Rate, Stated Percentage		4.65%
INA Senior Notes Due March 2034 and July 2034 [Domain]
Debt Instrument [Line Items]
Make Whole Premium Additional Percent	0.15%