Parent Company Cash Flow Statement (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Cash Flows from Operating Activities
Cash Provided by Operating Activities	$ 687.7	$ 1,513.2	$ 1,202.8
Cash Flows from Investing Activities
Proceeds from Maturities of Fixed Maturity Securities	2,131.7	1,483.0	1,578.1
Proceeds from Sales and Maturities of Other Investments	506.4	332.1	360.0
Purchase of Fixed Maturity Securities	(2,393.6)	(2,001.7)	(2,991.4)
Purchase of Other Investments	(361.2)	(319.1)	(361.2)
Net Purchases of Short-term Investments	(355.7)	(842.1)	(141.0)
Net Purchases of Property and Equipment	(132.3)	(125.7)	(134.8)
Net Cash Used by Investing Activities	548.9	(344.4)	(725.9)
Cash Flows from Financing Activities
Short-term Debt Repayment	(275.0)	0.0	(2.0)
Issuance of Long-term Debt	296.0	391.6	0.0
Long-term Debt Repayment	0.0	(350.0)	0.0
Issuance of Common Stock	5.1	6.0	5.2
Repurchase of Common Stock	(1,010.3)	(972.9)	(250.1)
Dividends Paid to Stockholders	(306.2)	(296.5)	(277.1)
Other, Net	(3.8)	(7.4)	(1.1)
Net Cash Used by Financing Activities	(1,241.2)	(1,152.0)	(450.1)
Net Increase (Decrease) in Cash and Bank Deposits	(4.6)	16.8	26.8
Parent Company
Cash Flows from Operating Activities
Cash Provided by Operating Activities	1,565.4	1,358.2	1,548.0
Cash Flows from Investing Activities
Proceeds from Maturities of Fixed Maturity Securities	269.1	378.0	97.1
Proceeds from Sales and Maturities of Other Investments	52.7	25.1	23.9
Purchase of Fixed Maturity Securities	(83.3)	(3.0)	(44.5)
Purchase of Other Investments	(38.9)	(16.1)	(23.4)
Net Purchases of Short-term Investments	(314.7)	(375.4)	(104.4)
Cash Distributions to Subsidiaries	(40.1)	(40.2)	(854.5)
Net Purchases of Property and Equipment	(99.7)	(91.1)	(113.1)
Net Cash Used by Investing Activities	(254.9)	(122.7)	(1,018.9)
Cash Flows from Financing Activities
Short-term Debt Repayment	(275.0)	0.0	(2.0)
Issuance of Long-term Debt	296.0	391.6	0.0
Long-term Debt Repayment	0.0	(350.0)	0.0
Issuance of Common Stock	5.1	6.0	5.2
Repurchase of Common Stock	(1,010.3)	(972.9)	(250.1)
Dividends Paid to Stockholders	(306.2)	(296.5)	(277.1)
Other, Net	(0.7)	(9.9)	0.0
Net Cash Used by Financing Activities	(1,291.1)	(1,231.7)	(524.0)
Net Increase (Decrease) in Cash and Bank Deposits	$ 19.4	$ 3.8	$ 5.1