Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Available-for-Sale Securities, Total Amortized Cost
1 year or less	$ 617.5	$ 715.1
Over 1 year through 5 years	5,281.0	5,161.5
Over 5 years through 10 years	9,659.8	9,630.5
Over 10 years	18,825.3	18,499.0
Available-for-sale Securities, Debt Maturities, Gross Subtotal	34,383.6	34,006.1
Mortgage/Asset-Backed Securities	2,504.4	2,634.6
Total Fixed Maturity Securities	36,888.0	36,640.7
Available-for-Sale Securities, Unrealized Gain Position, Gross Gain
1 year or less	17.3	16.0
Over 1 year through 5 years	496.2	449.8
Over 5 years through 10 years	1,315.4	1,266.3
Over 10 years	3,512.9	3,992.0
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Gain, Gross Subtotal, Total	5,341.8	5,724.1
Mortgage/Asset-Backed Securities	338.8	344.1
Total Fixed Maturity Securities	5,680.6	6,068.2
Available-for-Sale Securities, Unrealized Loss Position, Gross Loss
1 year or less	0	0.1
Over 1 year through 5 years	6.2	20.7
Over 5 years through 10 years	34.5	52.8
Over 10 years	115.8	143.1
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Loss, Gross Subtotal, Total	156.5	216.7
Mortgage/Asset-Backed Securities	0.9	5.5
Total Fixed Maturity Securities	157.4	222.2
Available-for-Sale Securities, Unrealized Gain Position, Fair Value
Fair Value Maturity Distribution [Abstract]
1 year or less	629.8	701.4
Over 1 year through 5 years	5,428.0	4,949.0
Over 5 years through 10 years	10,269.9	9,903.8
Over 10 years	20,874.5	21,082.2
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal, Total	37,202.2	36,636.4
Mortgage/Asset-Backed Securities	2,794.7	2,859.4
Total Fixed Maturity Securities	39,996.9	39,495.8
Available-for-Sale Securities, Unrealized Loss Position, Fair Value
Fair Value Maturity Distribution [Abstract]
1 year or less	5.0	29.6
Over 1 year through 5 years	343.0	641.6
Over 5 years through 10 years	670.8	940.2
Over 10 years	1,347.9	1,265.7
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal, Total	2,366.7	2,877.1
Mortgage/Asset-Backed Securities	47.6	113.8
Total Fixed Maturity Securities	$ 2,414.3	$ 2,990.9