Parent Company Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Assets
Fixed Maturity Securities - at fair value (amortized cost: $499.1; $545.0)	$ 33,056.6	$ 35,629.9
Other Long-term Investments	1,670.4	1,694.4
Short-term Investments	3,016.2	2,540.3
Deferred Income Tax	79.5	369.7
Other Assets	2,382.6	1,635.8
Total Assets	63,519.4	61,959.3
Liabilities
Short-term debt	0.0	274.6
Long-term Debt	3,767.6	3,465.2
Other Liabilities	2,995.8	2,650.1
Liabilities	52,400.3	50,998.2
Stockholders' Equity
Common Stock, $0.10 par; authorized: 725,000,000 shares; issued: 196,194,941 and 195,460,723 shares	19.6	19.5
Additional Paid-in Capital	1,593.0	1,489.6
Accumulated Other Comprehensive Loss	(1,808.5)	(2,523.7)
Retained Earnings	13,345.3	12,914.0
Treasury Stock - at cost: 30,500,524 and 16,871,752 shares	(2,030.3)	(938.3)
Total Stockholders' Equity	11,119.1	10,961.1	$ 9,651.4
Total Liabilities and Stockholders' Equity	63,519.4	61,959.3
Amortized Cost of Fixed Maturity Securities	34,747.2	38,269.9
Parent Company
Assets
Fixed Maturity Securities - at fair value (amortized cost: $499.1; $545.0)	472.7	497.4
Other Long-term Investments	32.7	32.7
Short-term Investments	1,766.6	1,397.5
Investments in Subsidiaries	12,721.6	12,982.0
Deferred Income Tax	77.0	122.5
Other Assets	683.2	620.0
Total Assets	15,753.8	15,652.1
Liabilities
Short-term debt	0.0	274.6
Long-term Debt	3,767.6	3,465.2
Pension and Postretirement Benefits	285.9	341.9
Other Liabilities	581.2	609.3
Liabilities	4,634.7	4,691.0
Stockholders' Equity
Common Stock, $0.10 par; authorized: 725,000,000 shares; issued: 196,194,941 and 195,460,723 shares	19.6	19.5
Additional Paid-in Capital	1,593.0	1,489.6
Accumulated Other Comprehensive Loss	(1,808.5)	(2,523.7)
Retained Earnings	13,345.3	12,914.0
Treasury Stock - at cost: 30,500,524 and 16,871,752 shares	(2,030.3)	(938.3)
Total Stockholders' Equity	11,119.1	10,961.1
Total Liabilities and Stockholders' Equity	15,753.8	15,652.1
Amortized Cost of Fixed Maturity Securities	$ 499.1	$ 545.0