Parent Company Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Condensed Balance Sheet Statements, Captions
Amortized Cost of Fixed Maturity Securities	$ 37,386.7	$ 36,546.5
Assets
Fixed Maturity Securities - at fair value (amortized cost: $349.5; $122.3)	43,336.0	44,137.3
Other Long-term Investments	1,203.0	960.2
Short-term Investments	1,388.0	1,470.0
Other Assets	1,825.6	1,752.3
Total Assets	70,115.6	70,625.8
Liabilities
Long-term Debt	3,442.2	3,345.7
Other Liabilities	2,585.9	2,471.4
Total Liabilities	58,699.2	59,754.8
Stockholders' Equity
Common Stock	30.7	30.7
Additional Paid-in Capital	2,408.1	2,376.2
Accumulated Other Comprehensive Income	354.1	374.2
Retained Earnings	11,853.2	11,269.6
Treasury Stock	(3,229.7)	(3,179.7)
Total Stockholders' Equity	11,416.4	10,871.0	$ 9,965.0
Total Liabilities and Stockholders' Equity	70,115.6	70,625.8
Parent Company
Condensed Balance Sheet Statements, Captions
Amortized Cost of Fixed Maturity Securities	826.4	349.5
Assets
Fixed Maturity Securities - at fair value (amortized cost: $349.5; $122.3)	820.8	355.1
Other Long-term Investments	10.3	10.6
Short-term Investments	629.5	164.5
Investments in Subsidiaries	13,711.1	14,122.6
Deferred Income Tax Assets, Net	144.5	158.8
Other Assets	508.1	496.7
Total Assets	15,824.3	15,308.3
Liabilities
Long-term Debt	3,442.2	3,345.7
Pension and Postretirement Benefits	507.1	677.5
Other Liabilities	458.6	414.1
Total Liabilities	4,407.9	4,437.3
Stockholders' Equity
Common Stock	30.7	30.7
Additional Paid-in Capital	2,408.1	2,376.2
Accumulated Other Comprehensive Income	354.1	374.2
Retained Earnings	11,853.2	11,269.6
Treasury Stock	(3,229.7)	(3,179.7)
Total Stockholders' Equity	11,416.4	10,871.0
Total Liabilities and Stockholders' Equity	$ 15,824.3	$ 15,308.3