Parent Company Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011	Dec. 31, 2010
Condensed Financial Statements, Captions [Line Items]
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis	$ 37,751.5		$ 36,640.7
Assets [Abstract]
Fixed Maturity Securities - at fair value (amortized cost: $217.7; $167.7)	44,973.0		42,486.7
Mortgage Loans	1,712.7	[1]	1,612.3
Short-term Investments	1,460.3		1,423.5
Other Assets	625.4		645.3
Total Assets	62,236.1		59,555.2
Liabilities
Short-term Debt	455.8		312.3
Long-term Debt	2,755.4		2,570.2
Other Liabilities	1,838.1		1,815.1
Total Liabilities	53,623.5		51,385.5
Common Stock	36.0		35.9
Additional Paid-in Capital	2,607.7		2,591.1
Retained Earnings	7,371.6		6,611.0
Treasury Stock	(2,030.7)		(1,530.1)
Total Stockholders' Equity	8,612.6		8,169.7	8,484.9
Total Liabilities and Stockholders' Equity	62,236.1		59,555.2
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis	217.7		167.7
Assets [Abstract]
Fixed Maturity Securities - at fair value (amortized cost: $217.7; $167.7)	224.6		172.7
Mortgage Loans	73.0		54.2
Short-term Investments	433.5		473.9
Investments in Subsidiaries	10,079.1		9,296.8
Other Assets	723.2		618.6
Total Assets	11,533.4		10,616.2
Liabilities
Short-term Debt	6.6		2.5
Long-term Debt	1,896.2		1,641.0
Pension and Other Postretirement Defined Benefit Plans, Liabilities	801.6		588.2
Other Liabilities	216.4		214.8
Total Liabilities	2,920.8		2,446.5
Common Stock	36.0		35.9
Additional Paid-in Capital	2,607.7		2,591.1
Accumulated Other Comprehensive Income (Loss), Net of Tax	628.0		461.8
Retained Earnings	7,371.6		6,611.0
Treasury Stock	(2,030.7)		(1,530.1)
Total Stockholders' Equity	8,612.6		8,169.7
Total Liabilities and Stockholders' Equity	$ 11,533.4		$ 10,616.2

[1] Amortized cost for fixed maturity securities and mortgage loans represents original cost reduced by repayments, write-downs from other-than-temporary declines in fair value, amortization of premiums, and accretion of discounts.