Parent Company Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Balance Sheet Statements, Captions
Amortized Cost of Fixed Maturity Securities	$ 36,546.5	$ 41,079.3
Assets
Fixed Maturity Securities - at fair value (amortized cost: $349.5; $122.3)	44,137.3	47,443.7
Other Long-term Investments	960.2	844.2
Short-term Investments	1,470.0	1,294.5
Other Assets	1,752.3	884.0
Total Assets	70,625.8	67,013.4
Liabilities
Short-term debt	0.0	399.7
Long-term Debt	3,345.7	2,926.9
Other Liabilities	2,471.4	1,880.5
Total Liabilities	59,754.8	57,048.4
Stockholders' Equity
Common Stock	30.7	30.6
Additional Paid-in Capital	2,376.2	2,348.1
Accumulated Other Comprehensive Income	374.2	37.3
Retained Earnings	11,269.6	10,728.7
Treasury Stock	(3,179.7)	(3,179.7)
Total Stockholders' Equity	10,871.0	9,965.0	$ 8,621.8
Total Liabilities and Stockholders' Equity	70,625.8	67,013.4
Parent Company
Condensed Balance Sheet Statements, Captions
Amortized Cost of Fixed Maturity Securities	349.5	122.3
Assets
Fixed Maturity Securities - at fair value (amortized cost: $349.5; $122.3)	355.1	125.6
Other Long-term Investments	10.6	35.2
Short-term Investments	164.5	604.4
Investments in Subsidiaries	14,122.6	12,820.9
Deferred Income Tax Assets, Net	158.8	122.7
Other Assets	496.7	488.9
Total Assets	15,308.3	14,197.7
Liabilities
Short-term debt	0.0	399.7
Long-term Debt	3,345.7	2,846.9
Pension and Postretirement Benefits	677.5	624.2
Other Liabilities	414.1	361.9
Total Liabilities	4,437.3	4,232.7
Stockholders' Equity
Common Stock	30.7	30.6
Additional Paid-in Capital	2,376.2	2,348.1
Accumulated Other Comprehensive Income	374.2	37.3
Retained Earnings	11,269.6	10,728.7
Treasury Stock	(3,179.7)	(3,179.7)
Total Stockholders' Equity	10,871.0	9,965.0
Total Liabilities and Stockholders' Equity	$ 15,308.3	$ 14,197.7