Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Available-for-Sale Securities, Total Amortized Cost
1 year or less	$ 841.9	$ 903.9
Over 1 year through 5 years	7,186.2	7,098.2
Over 5 years through 10 years	9,640.0	9,492.6
Over 10 years	18,745.4	18,936.2
Available-for-sale Securities, Debt Maturities, Gross Subtotal	36,413.5	36,430.9
Mortgage/Asset-Backed Securities	2,102.5	1,858.7
Total Fixed Maturity Securities	38,516.0	38,289.6
Fair Value Maturity Distribution
Total Fixed Maturity Securities	43,666.8	42,344.4
Fair Value of Fixed Maturity Securities in Unrealized Gain Position
Available-for-Sale Securities, Unrealized Gain Position, Gross Gain
1 year or less	21.5	20.6
Over 1 year through 5 years	769.6	727.1
Over 5 years through 10 years	1,030.0	940.2
Over 10 years	3,282.7	2,486.2
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Gain, Gross Subtotal	5,103.8	4,174.1
Mortgage/Asset-Backed Securities	186.8	184.6
Total Fixed Maturity Securities	5,290.6	4,358.7
Fair Value Maturity Distribution
1 year or less	854.4	915.5
Over 1 year through 5 years	7,908.9	7,678.5
Over 5 years through 10 years	9,239.5	8,137.4
Over 10 years	20,242.4	18,441.5
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	38,245.2	35,172.9
Mortgage/Asset-Backed Securities	2,022.9	1,695.8
Total Fixed Maturity Securities	40,268.1	36,868.7
Fair Value of Fixed Maturity Securities in Unrealized Loss Position
Fair Value Maturity Distribution
1 year or less	9.0	9.0
Over 1 year through 5 years	46.6	146.2
Over 5 years through 10 years	1,389.3	2,199.6
Over 10 years	1,690.0	2,777.4
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	3,134.9	5,132.2
Mortgage/Asset-Backed Securities	263.8	343.5
Total Fixed Maturity Securities	3,398.7	5,475.7
Available-for-Sale Securities, Unrealized Loss Position, Gross Loss
1 year or less	0	0
Over 1 year through 5 years	0.3	0.6
Over 5 years through 10 years	41.2	95.8
Over 10 years	95.7	203.5
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Loss, Gross Subtotal	137.2	299.9
Mortgage/Asset-Backed Securities	2.6	4.0
Total Fixed Maturity Securities	$ 139.8	$ 303.9