Parent Company Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Balance Sheet Statements, Captions
Amortized Cost of Fixed Maturity Securities	$ 41,079.3	$ 40,275.2
Assets
Fixed Maturity Securities - at fair value (amortized cost: $41,079.3; $40,275.2)	47,443.7	43,011.7
Other Long-term Investments	844.2	702.9
Short-term Investments	1,294.5	968.1
Deferred Income Tax Assets, Net	0.0	109.9
Other Assets	884.0	789.0
Total Assets	67,013.4	61,875.6
Liabilities
Short-term debt	399.7	0.0
Long-term Debt	2,926.9	2,971.3
Other Liabilities	1,880.5	1,762.8
Total Liabilities	57,048.4	53,253.8
Stockholders' Equity
Common Stock	30.6	30.5
Additional Paid-in Capital	2,348.1	2,321.7
Accumulated Other Comprehensive Income (Loss)	37.3	(814.2)
Retained Earnings	10,728.7	9,863.1
Treasury Stock	(3,179.7)	(2,779.3)
Total Stockholders' Equity	9,965.0	8,621.8	$ 9,574.9
Total Liabilities and Stockholders' Equity	67,013.4	61,875.6
Parent Company
Condensed Balance Sheet Statements, Captions
Amortized Cost of Fixed Maturity Securities	122.3	139.0
Assets
Fixed Maturity Securities - at fair value (amortized cost: $41,079.3; $40,275.2)	125.6	135.9
Other Long-term Investments	35.2	38.8
Short-term Investments	604.4	285.2
Investments in Subsidiaries	12,820.9	11,383.6
Deferred Income Tax Assets, Net	122.7	116.7
Other Assets	488.9	446.9
Total Assets	14,197.7	12,407.1
Liabilities
Short-term debt	399.7	0.0
Long-term Debt	2,846.9	2,831.3
Pension and Postretirement Benefits	624.2	594.2
Other Liabilities	361.9	359.8
Total Liabilities	4,232.7	3,785.3
Stockholders' Equity
Common Stock	30.6	30.5
Additional Paid-in Capital	2,348.1	2,321.7
Accumulated Other Comprehensive Income (Loss)	37.3	(814.2)
Retained Earnings	10,728.7	9,863.1
Treasury Stock	(3,179.7)	(2,779.3)
Total Stockholders' Equity	9,965.0	8,621.8
Total Liabilities and Stockholders' Equity	$ 14,197.7	$ 12,407.1