Parent Company Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Fixed Maturity Securities - at fair value (amortized cost: $215.1; $273.7)	$ 44,217.3	$ 43,354.4
Other Long-term Investments	631.5	583.0
Short-term Investments	780.0	807.3
Other Assets	617.3	637.4
Total Assets	61,941.5	60,563.6
Liabilities
Short-term Debt	0.0	352.0
Long-term Debt	2,999.4	2,449.4
Other Liabilities	1,675.9	1,501.1
Total Liabilities	52,973.5	51,899.7
Stockholders' Equity
Common Stock	30.4	30.3
Additional Paid-in Capital	2,272.8	2,247.2
Accumulated Other Comprehensive Income (Loss)	(51.0)	16.1
Retained Earnings	8,744.0	7,995.2
Treasury Stock	(2,028.2)	(1,624.9)
Total Stockholders' Equity	8,968.0	8,663.9	$ 8,521.9
Total Liabilities and Stockholders' Equity	61,941.5	60,563.6
Parent Company
Assets
Fixed Maturity Securities - at fair value (amortized cost: $215.1; $273.7)	214.2	273.4
Other Long-term Investments	49.0	22.7
Short-term Investments	377.9	288.1
Investments in Subsidiaries	11,340.7	10,689.1
Deferred Income Tax	210.7	160.1
Other Assets	473.9	546.4
Total Assets	12,666.4	11,979.8
Liabilities
Short-term Debt	0.0	352.0
Long-term Debt	2,735.9	2,125.4
Pension and Postretirement Benefits	638.3	554.1
Other Liabilities	324.2	284.4
Total Liabilities	3,698.4	3,315.9
Stockholders' Equity
Common Stock	30.4	30.3
Additional Paid-in Capital	2,272.8	2,247.2
Accumulated Other Comprehensive Income (Loss)	(51.0)	16.1
Retained Earnings	8,744.0	7,995.2
Treasury Stock	(2,028.2)	(1,624.9)
Total Stockholders' Equity	8,968.0	8,663.9
Total Liabilities and Stockholders' Equity	$ 12,666.4	$ 11,979.8