Parent Company Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Assets [Abstract]
Fixed Maturity Securities	$ 45,064.9	$ 42,344.4
Other Long-term Investments	591.9	566.0
Short-term Investments	974.3	913.4
Other Assets	661.9	647.8
Total Assets	62,497.1	59,403.6
Liabilities
Long-term Debt	2,628.7	2,612.0
Other Liabilities	1,572.6	1,305.9
Total Liabilities	53,944.7	50,744.5
Stockholders' Equity
Common Stock	30.2	36.1
Additional Paid-in Capital	2,221.2	2,634.1
Accumulated Other Comprehensive Income	166.4	255.0	628.0
Retained Earnings	7,332.8	8,083.2
Treasury Stock	(1,198.2)	(2,349.3)
Total Stockholders' Equity	8,552.4	8,659.1	8,612.6
Total Liabilities and Stockholders' Equity	62,497.1	59,403.6
Parent Company
Assets [Abstract]
Fixed Maturity Securities	162.2	145.3
Other Long-term Investments	51.9	57.2
Short-term Investments	326.2	164.0
Investments in Subsidiaries	10,362.7	10,082.8
Deferred Income Tax	160.1	68.1
Other Assets	568.3	558.8
Total Assets	11,631.4	11,076.2
Liabilities
Long-term Debt	2,230.3	1,875.2
Pension and Postretirement Benefits	581.5	295.3
Other Liabilities	267.2	246.6
Total Liabilities	3,079.0	2,417.1
Stockholders' Equity
Common Stock	30.2	36.1
Additional Paid-in Capital	2,221.2	2,634.1
Accumulated Other Comprehensive Income	166.4	255.0
Retained Earnings	7,332.8	8,083.2
Treasury Stock	(1,198.2)	(2,349.3)
Total Stockholders' Equity	8,552.4	8,659.1
Total Liabilities and Stockholders' Equity	$ 11,631.4	$ 11,076.2