Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Available-for-Sale Securities, Total Amortized Cost
1 year or less	$ 956.4	$ 715.1
Over 1 year through 5 years	5,922.8	5,161.5
Over 5 years through 10 years	9,752.3	9,630.5
Over 10 years	19,192.1	18,499.0
Available-for-sale Securities, Debt Maturities, Gross Subtotal	35,823.6	34,006.1
Mortgage/Asset-Backed Securities	1,927.9	2,634.6
Total Fixed Maturity Securities	37,751.5	36,640.7
Available-for-Sale Securities, Unrealized Gain Position, Gross Gain
1 year or less	21.2	16.0
Over 1 year through 5 years	628.1	449.8
Over 5 years through 10 years	1,606.4	1,266.3
Over 10 years	4,732.7	3,992.0
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Gain, Gross Subtotal, Total	6,988.4	5,724.1
Mortgage/Asset-Backed Securities	289.1	344.1
Total Fixed Maturity Securities	7,277.5	6,068.2
Available-for-Sale Securities, Unrealized Loss Position, Gross Loss
1 year or less	0.5	0.1
Over 1 year through 5 years	5.0	20.7
Over 5 years through 10 years	7.6	52.8
Over 10 years	42.4	143.1
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Loss, Gross Subtotal, Total	55.5	216.7
Mortgage/Asset-Backed Securities	0.5	5.5
Total Fixed Maturity Securities	56.0	222.2
Available-for-Sale Securities, Unrealized Gain Position, Fair Value
Fair Value Maturity Distribution [Abstract]
1 year or less	934.1	701.4
Over 1 year through 5 years	6,449.8	4,949.0
Over 5 years through 10 years	10,997.0	9,903.8
Over 10 years	23,200.2	21,082.2
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal, Total	41,581.1	36,636.4
Mortgage/Asset-Backed Securities	2,208.3	2,859.4
Fair value of fixed maturity securities	43,789.4	39,495.8
Available-for-Sale Securities, Unrealized Loss Position, Fair Value
Fair Value Maturity Distribution [Abstract]
1 year or less	43.0	29.6
Over 1 year through 5 years	96.1	641.6
Over 5 years through 10 years	354.1	940.2
Over 10 years	682.2	1,265.7
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal, Total	1,175.4	2,877.1
Mortgage/Asset-Backed Securities	8.2	113.8
Fair value of fixed maturity securities	$ 1,183.6	$ 2,990.9