Parent Company Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Other Long-term Investments	$ 702.9	$ 646.8
Short-term Investments	968.1	1,155.1
Deferred Income Tax	109.9
Other Assets	789.0	628.7
Total Assets	61,875.6	64,013.1
Liabilities
Short-term Debt	0.0	199.9
Long-term Debt	2,971.3	2,738.4
Other Liabilities	1,634.7	1,726.9
Total Liabilities	53,253.8	54,438.2
Stockholders' Equity
Common Stock	30.5	30.5
Additional Paid-in Capital	2,321.7	2,303.3
Accumulated Other Comprehensive Income (Loss)	(814.2)	127.5
Retained Earnings	9,863.1	9,542.2
Treasury Stock	(2,779.3)	(2,428.6)
Total Stockholders' Equity	8,621.8	9,574.9	$ 8,968.0
Total Liabilities and Stockholders' Equity	61,875.6	64,013.1
Parent Company
Assets
Fixed Maturity Securities - at fair value (amortized cost: $139.0; $153.9)	135.9	154.1
Other Long-term Investments	38.8	16.8
Short-term Investments	285.2	477.8
Investments in Subsidiaries	11,383.6	12,157.1
Deferred Income Tax	116.7	121.5
Other Assets	446.9	455.4
Total Assets	12,407.1	13,382.7
Liabilities
Short-term Debt	0.0	199.9
Long-term Debt	2,831.3	2,538.4
Pension and Postretirement Benefits	594.2	683.3
Other Liabilities	359.8	386.2
Total Liabilities	3,785.3	3,807.8
Stockholders' Equity
Common Stock	30.5	30.5
Additional Paid-in Capital	2,321.7	2,303.3
Accumulated Other Comprehensive Income (Loss)	(814.2)	127.5
Retained Earnings	9,863.1	9,542.2
Treasury Stock	(2,779.3)	(2,428.6)
Total Stockholders' Equity	8,621.8	9,574.9
Total Liabilities and Stockholders' Equity	$ 12,407.1	$ 13,382.7