Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Available-for-Sale Securities, Total Amortized Cost
1 year or less	$ 1,059.7	$ 956.4
Over 1 year through 5 years	5,927.8	5,922.8
Over 5 years through 10 years	9,756.5	9,752.3
Over 10 years	19,121.5	19,192.1
Available-for-sale Securities, Debt Maturities, Gross Subtotal	35,865.5	35,823.6
Mortgage/Asset-Backed Securities	1,767.1	1,927.9
Total Fixed Maturity Securities	37,632.6	37,751.5
Available-for-Sale Securities, Unrealized Gain Position, Gross Gain
1 year or less	23.6	21.2
Over 1 year through 5 years	654.8	628.1
Over 5 years through 10 years	1,548.7	1,606.4
Over 10 years	4,347.5	4,732.7
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Gain, Gross Subtotal, Total	6,574.6	6,988.4
Mortgage/Asset-Backed Securities	275.6	289.1
Total Fixed Maturity Securities	6,850.2	7,277.5
Available-for-Sale Securities, Unrealized Loss Position, Gross Loss
1 year or less	0.2	0.5
Over 1 year through 5 years	1.7	5.0
Over 5 years through 10 years	8.0	7.6
Over 10 years	35.9	42.4
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Loss, Gross Subtotal, Total	45.8	55.5
Mortgage/Asset-Backed Securities	0.5	0.5
Total Fixed Maturity Securities	46.3	56.0
Available-for-Sale Securities, Unrealized Gain Position, Fair Value
Fair Value Maturity Distribution [Abstract]
1 year or less	1,040.1	934.1
Over 1 year through 5 years	6,501.5	6,449.8
Over 5 years through 10 years	10,799.5	10,997.0
Over 10 years	22,461.7	23,200.2
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal, Total	40,802.8	41,581.1
Mortgage/Asset-Backed Securities	2,035.9	2,208.3
Fair value of fixed maturity securities	42,838.7	43,789.4
Available-for-Sale Securities, Unrealized Loss Position, Fair Value
Fair Value Maturity Distribution [Abstract]
1 year or less	43.0	43.0
Over 1 year through 5 years	79.4	96.1
Over 5 years through 10 years	497.7	354.1
Over 10 years	971.4	682.2
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal, Total	1,591.5	1,175.4
Mortgage/Asset-Backed Securities	6.3	8.2
Fair value of fixed maturity securities	$ 1,597.8	$ 1,183.6