Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Available-for-Sale Securities, Total Amortized Cost
1 year or less	$ 1,165.8	$ 903.9
Over 1 year through 5 years	6,938.3	7,098.2
Over 5 years through 10 years	9,566.8	9,492.6
Over 10 years	18,939.7	18,936.2
Available-for-sale Securities, Debt Maturities, Gross Subtotal	36,610.6	36,430.9
Mortgage/Asset-Backed Securities	2,309.8	1,858.7
Total Fixed Maturity Securities	38,920.4	38,289.6
Fair Value Maturity Distribution
Total Fixed Maturity Securities	44,715.2	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	29.5	20.6
Over 1 year through 5 years	749.6	727.1
Over 5 years through 10 years	996.7	940.2
Over 10 years	3,911.1	2,486.2
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Gain, Gross Subtotal	5,686.9	4,174.1
Mortgage/Asset-Backed Securities	192.5	184.6
Total Fixed Maturity Securities	5,879.4	4,358.7
Fair Value Maturity Distribution
1 year or less	1,195.3	915.5
Over 1 year through 5 years	7,642.5	7,678.5
Over 5 years through 10 years	9,052.8	8,137.4
Over 10 years	21,662.8	18,441.5
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	39,553.4	35,172.9
Mortgage/Asset-Backed Securities	2,476.3	1,695.8
Total Fixed Maturity Securities	42,029.7	36,868.7
Fair Value of Fixed Maturity Securities in Unrealized Loss Position
Fair Value Maturity Distribution
1 year or less	0	9.0
Over 1 year through 5 years	45.1	146.2
Over 5 years through 10 years	1,467.2	2,199.6
Over 10 years	1,147.8	2,777.4
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	2,660.1	5,132.2
Mortgage/Asset-Backed Securities	25.4	343.5
Total Fixed Maturity Securities	2,685.5	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	43.5	95.8
Over 10 years	40.2	203.5
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Loss, Gross Subtotal	84.0	299.9
Mortgage/Asset-Backed Securities	0.6	4.0
Total Fixed Maturity Securities	$ 84.6	$ 303.9