Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Available-for-Sale Securities, Total Amortized Cost
1 year or less	$ 903.9	$ 956.4
Over 1 year through 5 years	7,098.2	5,922.8
Over 5 years through 10 years	9,492.6	9,752.3
Over 10 years	18,936.2	19,192.1
Available-for-sale Securities, Debt Maturities, Gross Subtotal	36,430.9	35,823.6
Mortgage/Asset-Backed Securities	1,858.7	1,927.9
Total Fixed Maturity Securities	38,289.6	37,751.5
Fair Value Maturity Distribution [Abstract]
Total Fixed Maturity Securities	42,344.4	44,973.0
Fair Value of Fixed Maturity Securities in Unrealized Gain Position
Available-for-Sale Securities, Unrealized Gain Position, Gross Gain
1 year or less	20.6	21.2
Over 1 year through 5 years	727.1	628.1
Over 5 years through 10 years	940.2	1,606.4
Over 10 years	2,486.2	4,732.7
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Gain, Gross Subtotal	4,174.1	6,988.4
Mortgage/Asset-Backed Securities	184.6	289.1
Total Fixed Maturity Securities	4,358.7	7,277.5
Fair Value Maturity Distribution [Abstract]
1 year or less	915.5	934.1
Over 1 year through 5 years	7,678.5	6,449.8
Over 5 years through 10 years	8,137.4	10,997.0
Over 10 years	18,441.5	23,200.2
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	35,172.9	41,581.1
Mortgage/Asset-Backed Securities	1,695.8	2,208.3
Total Fixed Maturity Securities	36,868.7	43,789.4
Fair Value of Fixed Maturity Securities in Unrealized Loss Position
Fair Value Maturity Distribution [Abstract]
1 year or less	9.0	43.0
Over 1 year through 5 years	146.2	96.1
Over 5 years through 10 years	2,199.6	354.1
Over 10 years	2,777.4	682.2
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	5,132.2	1,175.4
Mortgage/Asset-Backed Securities	343.5	8.2
Total Fixed Maturity Securities	5,475.7	1,183.6
Available-for-Sale Securities, Unrealized Loss Position, Gross Loss
1 year or less	0	0.5
Over 1 year through 5 years	0.6	5.0
Over 5 years through 10 years	95.8	7.6
Over 10 years	203.5	42.4
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Loss, Gross Subtotal	299.9	55.5
Mortgage/Asset-Backed Securities	4.0	0.5
Total Fixed Maturity Securities	$ 303.9	$ 56.0