Parent Company Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Assets
Fixed Maturity Securities - at fair value (amortized cost: $545.0; $656.2)	$ 35,629.9	$ 36,833.9
Other Long-term Investments	1,694.4	1,579.4
Short-term Investments	2,540.3	1,610.7
Deferred Income Tax	369.7	649.4
Other Assets	1,635.8	1,661.7
Total Assets	61,959.3	63,255.2
Liabilities
Short-term debt	274.6	0.0
Long-term Debt	3,465.2	3,430.4
Other Liabilities	2,650.1	2,283.4
Liabilities	50,998.2	53,603.8
Stockholders' Equity
Common Stock, $0.10 par; authorized: 725,000,000 shares; issued: 195,460,723 and 194,588,625 shares	19.5	19.4
Additional Paid-in Capital	1,489.6	1,547.8
Accumulated Other Comprehensive Loss	(2,523.7)	(3,308.0)
Retained Earnings	12,914.0	11,431.5
Treasury Stock - at cost: 16,871,752 and 1,216,528 shares	(938.3)	(39.3)
Total Stockholders' Equity	10,961.1	9,651.4	$ 8,735.0
Total Liabilities and Stockholders' Equity	61,959.3	63,255.2
Amortized Cost of Fixed Maturity Securities	38,269.9	38,410.6
Parent Company
Assets
Fixed Maturity Securities - at fair value (amortized cost: $545.0; $656.2)	497.4	596.1
Other Long-term Investments	32.7	27.0
Short-term Investments	1,397.5	970.9
Investments in Subsidiaries	12,982.0	11,663.6
Deferred Income Tax	122.5	118.1
Other Assets	620.0	593.6
Total Assets	15,652.1	13,969.3
Liabilities
Short-term debt	274.6	0.0
Long-term Debt	3,465.2	3,430.4
Pension and Postretirement Benefits	341.9	350.2
Other Liabilities	609.3	537.3
Liabilities	4,691.0	4,317.9
Stockholders' Equity
Common Stock, $0.10 par; authorized: 725,000,000 shares; issued: 195,460,723 and 194,588,625 shares	19.5	19.4
Additional Paid-in Capital	1,489.6	1,547.8
Accumulated Other Comprehensive Loss	(2,523.7)	(3,308.0)
Retained Earnings	12,914.0	11,431.5
Treasury Stock - at cost: 16,871,752 and 1,216,528 shares	(938.3)	(39.3)
Total Stockholders' Equity	10,961.1	9,651.4
Total Liabilities and Stockholders' Equity	15,652.1	13,969.3
Amortized Cost of Fixed Maturity Securities	$ 545.0	$ 656.2