Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Available-for-Sale Securities, Total Amortized Cost
1 year or less	$ 990.2	$ 715.1
Over 1 year through 5 years	5,613.8	5,161.5
Over 5 years through 10 years	9,776.2	9,630.5
Over 10 years	18,971.8	18,499.0
Available-for-sale Securities, Debt Maturities, Gross Subtotal	35,352.0	34,006.1
Mortgage/Asset-Backed Securities	2,105.3	2,634.6
Total Fixed Maturity Securities	37,457.3	36,640.7
Available-for-Sale Securities, Unrealized Gain Position, Gross Gain
1 year or less	23.4	16.0
Over 1 year through 5 years	590.6	449.8
Over 5 years through 10 years	1,623.1	1,266.3
Over 10 years	4,822.5	3,992.0
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Gain, Gross Subtotal, Total	7,059.6	5,724.1
Mortgage/Asset-Backed Securities	318.8	344.1
Total Fixed Maturity Securities	7,378.4	6,068.2
Available-for-Sale Securities, Unrealized Loss Position, Gross Loss
1 year or less	0.1	0.1
Over 1 year through 5 years	7.6	20.7
Over 5 years through 10 years	17.0	52.8
Over 10 years	65.8	143.1
Available-For-Sale Securities, Debt Maturities Unrealized Loss Position, Gross Loss, Gross Subtotal, Total	90.5	216.7
Mortgage/Asset-Backed Securities	0.5	5.5
Total Fixed Maturity Securities	91.0	222.2
Available-for-Sale Securities, Unrealized Gain Position, Fair Value
Fair Value Maturity Distribution [Abstract]
1 year or less	988.7	701.4
Over 1 year through 5 years	6,014.8	4,949.0
Over 5 years through 10 years	11,051.4	9,903.8
Over 10 years	23,082.5	21,082.2
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal, Total	41,137.4	36,636.4
Mortgage/Asset-Backed Securities	2,415.3	2,859.4
Total Fixed Maturity Securities	43,552.7	39,495.8
Available-for-Sale Securities, Unrealized Loss Position, Fair Value
Fair Value Maturity Distribution [Abstract]
1 year or less	24.8	29.6
Over 1 year through 5 years	182.0	641.6
Over 5 years through 10 years	330.9	940.2
Over 10 years	646.0	1,265.7
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal, Total	1,183.7	2,877.1
Mortgage/Asset-Backed Securities	8.3	113.8
Total Fixed Maturity Securities	$ 1,192.0	$ 2,990.9