Parent Company Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Assets
Fixed Maturity Securities	$ 43,354.4	$ 45,064.9
Other Long-term Investments	583.0	545.0
Short-term Investments	807.3	974.3
Other Assets	663.5	661.9
Total Assets	60,589.7	62,450.2
Liabilities
Short-term Debt	352.4	151.9
Long-term Debt	2,475.1	2,628.7
Other Liabilities	1,501.1	1,498.8
Total Liabilities	51,925.8	53,928.3
Stockholders' Equity
Common Stock	30.3	30.2
Additional Paid-in Capital	2,247.2	2,221.2
Accumulated Other Comprehensive Income	16.1	166.4
Retained Earnings	7,995.2	7,302.3	$ 8,064.0	$ 7,363.5
Treasury Stock	(1,624.9)	(1,198.2)
Total Stockholders' Equity	8,663.9	8,521.9	$ 8,639.9
Total Liabilities and Stockholders' Equity	60,589.7	62,450.2
Parent Company
Assets
Fixed Maturity Securities	273.4	162.2
Other Long-term Investments	22.7	51.9
Short-term Investments	288.1	326.2
Investments in Subsidiaries	10,689.1	10,332.2
Deferred Income Tax	160.1	160.1
Other Assets	572.5	568.3
Total Assets	12,005.9	11,600.9
Liabilities
Short-term Debt	352.4	0.0
Long-term Debt	2,151.1	2,230.3
Pension and Postretirement Benefits	554.1	581.5
Other Liabilities	284.4	267.2
Total Liabilities	3,342.0	3,079.0
Stockholders' Equity
Common Stock	30.3	30.2
Additional Paid-in Capital	2,247.2	2,221.2
Accumulated Other Comprehensive Income	16.1	166.4
Retained Earnings	7,995.2	7,302.3
Treasury Stock	(1,624.9)	(1,198.2)
Total Stockholders' Equity	8,663.9	8,521.9
Total Liabilities and Stockholders' Equity	$ 12,005.9	$ 11,600.9