Parent Company Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Condensed Balance Sheet Statements, Captions
Amortized Cost of Fixed Maturity Securities	$ 37,825.2	$ 37,386.7
Assets
Fixed Maturity Securities - at fair value (amortized cost: $748.1; $826.4)	34,840.8	43,336.0
Other Long-term Investments	1,440.1	1,203.0
Short-term Investments	1,394.8	1,388.0
Deferred Income Tax Assets, Net	449.8	0.0
Other Assets	1,786.3	1,825.6
Total Assets	61,434.9	70,115.6
Liabilities
Short-term debt	2.0	0.0
Long-term Debt	3,427.8	3,442.2
Other Liabilities	2,334.4	2,585.9
Total Liabilities	52,237.4	58,699.2
Stockholders' Equity
Common Stock	30.8	30.7
Additional Paid-in Capital	2,441.0	2,408.1
Accumulated Other Comprehensive Income	(2,756.6)	354.1
Retained Earnings	12,912.1	11,853.2
Treasury Stock	(3,429.8)	(3,229.7)
Total Stockholders' Equity	9,197.5	11,416.4	$ 10,871.0
Total Liabilities and Stockholders' Equity	61,434.9	70,115.6
Parent Company
Condensed Balance Sheet Statements, Captions
Amortized Cost of Fixed Maturity Securities	748.1	826.4
Assets
Fixed Maturity Securities - at fair value (amortized cost: $748.1; $826.4)	657.7	820.8
Other Long-term Investments	19.3	10.3
Short-term Investments	845.8	629.5
Investments in Subsidiaries	11,278.2	13,711.1
Deferred Income Tax Assets, Net	114.9	144.5
Other Assets	555.5	508.1
Total Assets	13,471.4	15,824.3
Liabilities
Short-term debt	2.0	0.0
Long-term Debt	3,427.8	3,442.2
Pension and Postretirement Benefits	352.6	507.1
Other Liabilities	491.5	458.6
Total Liabilities	4,273.9	4,407.9
Stockholders' Equity
Common Stock	30.8	30.7
Additional Paid-in Capital	2,441.0	2,408.1
Accumulated Other Comprehensive Income	(2,756.6)	354.1
Retained Earnings	12,912.1	11,853.2
Treasury Stock	(3,429.8)	(3,229.7)
Total Stockholders' Equity	9,197.5	11,416.4
Total Liabilities and Stockholders' Equity	$ 13,471.4	$ 15,824.3