Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Available-for-Sale Securities, Total Amortized Cost
1 year or less	$ 638.1	$ 715.1
Over 1 year through 5 years	5,782.4	5,161.5
Over 5 years through 10 years	9,394.2	9,630.5
Over 10 years	19,011.5	18,499.0
Available-for-sale Securities, Debt Maturities, Gross Subtotal	34,826.2	34,006.1
Mortgage/Asset-Backed Securities	2,285.8	2,634.6
Total Fixed Maturity Securities	37,112.0	36,640.7
Available-for-Sale Securities, Unrealized Gain Position, Gross Gain
1 year or less	14.3	16.0
Over 1 year through 5 years	538.6	449.8
Over 5 years through 10 years	1,371.5	1,266.3
Over 10 years	4,328.9	3,992.0
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Gain, Gross Subtotal, Total	6,253.3	5,724.1
Mortgage/Asset-Backed Securities	319.4	344.1
Total Fixed Maturity Securities	6,572.7	6,068.2
Available-for-Sale Securities, Unrealized Loss Position, Gross Loss
1 year or less	0.2	0.1
Over 1 year through 5 years	10.5	20.7
Over 5 years through 10 years	39.6	52.8
Over 10 years	117.0	143.1
Available-For-Sale Securities, Debt Maturities Unrealized Loss Position, Gross Loss, Gross Subtotal, Total	167.3	216.7
Mortgage/Asset-Backed Securities	0.5	5.5
Total Fixed Maturity Securities	167.8	222.2
Available-for-Sale Securities, Unrealized Gain Position, Fair Value
Fair Value Maturity Distribution [Abstract]
1 year or less	637.4	701.4
Over 1 year through 5 years	5,983.3	4,949.0
Over 5 years through 10 years	10,072.9	9,903.8
Over 10 years	21,993.6	21,082.2
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal, Total	38,687.2	36,636.4
Mortgage/Asset-Backed Securities	2,591.4	2,859.4
Total Fixed Maturity Securities	41,278.6	39,495.8
Available-for-Sale Securities, Unrealized Loss Position, Fair Value
Fair Value Maturity Distribution [Abstract]
1 year or less	14.8	29.6
Over 1 year through 5 years	327.2	641.6
Over 5 years through 10 years	653.2	940.2
Over 10 years	1,229.8	1,265.7
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal, Total	2,225.0	2,877.1
Mortgage/Asset-Backed Securities	13.3	113.8
Total Fixed Maturity Securities	$ 2,238.3	$ 2,990.9