Parent Company Cash Flow Statement (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash Flows from Operating Activities
Cash Provided by Operating Activities	$ 1,202.8	$ 1,418.7	$ 1,387.5
Cash Flows from Investing Activities
Proceeds from Maturities of Fixed Maturity Securities	1,578.1	1,532.5	2,611.4
Proceeds from Sales and Maturities of Other Investments	360.0	445.9	403.1
Purchase of Fixed Maturity Securities	(2,991.4)	(2,835.3)	(4,106.3)
Purchase of Other Investments	(361.2)	(482.9)	(606.8)
Net Sales (Purchases) of Short-term Investments	(141.0)	(22.6)	76.5
Net Purchases of Property and Equipment	(134.8)	(102.2)	(110.3)
Cash Used by Investing Activities	(725.9)	(955.9)	(1,340.6)
Cash Flows from Financing Activities
Short-term Debt Repayment	(2.0)	0.0	0.0
Issuance of Long-term Debt	0.0	349.2	588.1
Long-term Debt Repayment	0.0	(364.0)	(500.0)
Cost Related to Early Retirement of Debt	0.0	3.6	62.8
Issuance of Common Stock	5.2	4.0	3.4
Repurchase of Common Stock	(250.1)	(200.1)	(50.0)
Dividends Paid to Stockholders	(277.1)	(254.2)	(239.4)
Other, Net	(1.1)	(2.9)	4.0
Cash Used by Financing Activities	(450.1)	(418.6)	(168.9)
Net Increase (Decrease) in Cash and Bank Deposits	26.8	44.2	(122.0)
Parent Company
Cash Flows from Operating Activities
Cash Provided by Operating Activities	1,548.0	1,250.2	861.0
Cash Flows from Investing Activities
Proceeds from Maturities of Fixed Maturity Securities	97.1	192.1	245.5
Proceeds from Sales and Maturities of Other Investments	23.9	7.4	20.1
Purchase of Fixed Maturity Securities	(44.5)	(102.7)	(2.0)
Purchase of Other Investments	(23.4)	(32.0)	(0.9)
Net Sales (Purchases) of Short-term Investments	(104.4)	(209.6)	(465.0)
Cash Distributions to Subsidiaries	(854.5)	(540.2)	(300.9)
Net Purchases of Property and Equipment	(113.1)	(94.0)	(91.3)
Cash Used by Investing Activities	(1,018.9)	(779.0)	(594.5)
Cash Flows from Financing Activities
Short-term Debt Repayment	(2.0)	0.0	0.0
Issuance of Long-term Debt	0.0	349.2	588.1
Long-term Debt Repayment	0.0	(364.0)	(500.0)
Cost Related to Early Retirement of Debt	0.0	3.6	62.8
Issuance of Common Stock	5.2	4.0	3.4
Repurchase of Common Stock	(250.1)	(200.1)	(50.0)
Dividends Paid to Stockholders	(277.1)	(254.2)	(239.4)
Other, Net	0.0	0.7	(1.6)
Cash Used by Financing Activities	(524.0)	(468.0)	(262.3)
Net Increase (Decrease) in Cash and Bank Deposits	$ 5.1	$ 3.2	$ 4.2