Parent Company Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Fixed Maturity Securities - at fair value (amortized cost: $153.9; $215.1)	$ 45,457.8	$ 44,217.3
Other Long-term Investments	646.8	631.5
Short-term Investments	1,155.1	780.0
Other Assets	628.7	617.3
Total Assets	64,013.1	61,941.5
Liabilities
Short-term Debt	199.9	0.0
Long-term Debt	2,738.4	2,999.4
Other Liabilities	1,726.9	1,675.9
Total Liabilities	54,438.2	52,973.5
Stockholders' Equity
Common Stock	30.5	30.4
Additional Paid-in Capital	2,303.3	2,272.8
Accumulated Other Comprehensive Income (Loss)	127.5	(51.0)
Retained Earnings	9,542.2	8,744.0
Treasury Stock	(2,428.6)	(2,028.2)
Total Stockholders' Equity	9,574.9	8,968.0	$ 8,663.9
Total Liabilities and Stockholders' Equity	64,013.1	61,941.5
Parent Company
Assets
Fixed Maturity Securities - at fair value (amortized cost: $153.9; $215.1)	154.1	214.2
Other Long-term Investments	16.8	49.0
Short-term Investments	477.8	377.9
Investments in Subsidiaries	12,157.1	11,340.7
Deferred Income Tax	121.5	210.7
Other Assets	455.4	473.9
Total Assets	13,382.7	12,666.4
Liabilities
Short-term Debt	199.9	0.0
Long-term Debt	2,538.4	2,735.9
Pension and Postretirement Benefits	683.3	638.3
Other Liabilities	386.2	324.2
Total Liabilities	3,807.8	3,698.4
Stockholders' Equity
Common Stock	30.5	30.4
Additional Paid-in Capital	2,303.3	2,272.8
Accumulated Other Comprehensive Income (Loss)	127.5	(51.0)
Retained Earnings	9,542.2	8,744.0
Treasury Stock	(2,428.6)	(2,028.2)
Total Stockholders' Equity	9,574.9	8,968.0
Total Liabilities and Stockholders' Equity	$ 13,382.7	$ 12,666.4