Parent Company Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021	Jan. 01, 2021
Assets
Fixed Maturity Securities - at fair value (amortized cost: $656.2; $748.1)	$ 36,833.9	$ 34,840.8
Other Long-term Investments	1,579.4	1,440.1
Short-term Investments	1,610.7	1,394.8
Deferred Income Tax Assets, Net	649.4	586.0
Other Assets	1,661.7	1,666.6	[1]
Total Assets	63,255.2	61,148.5
Liabilities
Short-term debt	0.0	2.0
Long-term Debt	3,430.4	3,427.8
Other Liabilities	2,283.4	2,334.4
Liabilities	53,603.8	52,413.5
Stockholders' Equity
Common Stock	19.4	30.8
Additional Paid-in Capital	1,547.8	2,441.0
Accumulated Other Comprehensive Income	(3,308.0)	(3,448.3)		$ (5,164.6)	$ (6,310.2)
Retained Earnings	11,431.5	13,141.3		11,989.4	$ 11,249.0
Treasury Stock	(39.3)	(3,429.8)
Total Stockholders' Equity	9,651.4	8,735.0		$ 6,033.9
Total Liabilities and Stockholders' Equity	63,255.2	61,148.5
Amortized Cost of Fixed Maturity Securities	38,410.6	37,825.2
Parent Company
Assets
Fixed Maturity Securities - at fair value (amortized cost: $656.2; $748.1)	596.1	657.7
Other Long-term Investments	27.0	19.3
Short-term Investments	970.9	845.8
Investments in Subsidiaries	11,663.6	10,815.7
Deferred Income Tax Assets, Net	118.1	114.9
Other Assets	593.6	555.5
Total Assets	13,969.3	13,008.9
Liabilities
Short-term debt	0.0	2.0
Long-term Debt	3,430.4	3,427.8
Pension and Postretirement Benefits	350.2	352.6
Other Liabilities	537.3	491.5
Liabilities	4,317.9	4,273.9
Stockholders' Equity
Common Stock	19.4	30.8
Additional Paid-in Capital	1,547.8	2,441.0
Accumulated Other Comprehensive Income	(3,308.0)	(3,448.3)
Retained Earnings	11,431.5	13,141.3
Treasury Stock	(39.3)	(3,429.8)
Total Stockholders' Equity	9,651.4	8,735.0
Total Liabilities and Stockholders' Equity	13,969.3	13,008.9
Amortized Cost of Fixed Maturity Securities	$ 656.2	$ 748.1

[1] 1The change in other assets is driven by the cost of reinsurance associated with the second phase of our Closed Block individual disability reinsurance transaction which was completed in the first quarter of 2021. In accordance with the provisions of the ASU related to non-contemporaneous reinsurance, we were then required to establish the ceded reserves using an upper-medium grade fixed-income instrument as of the reinsurance transaction date in March 2021 which resulted in higher ceded reserves compared to that which was reported historically.