Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Available-for-Sale Securities, Total Amortized Cost
1 year or less	$ 1,372.0	$ 903.9
Over 1 year through 5 years	6,871.2	7,098.2
Over 5 years through 10 years	9,532.9	9,492.6
Over 10 years	18,802.4	18,936.2
Available-for-sale Securities, Debt Maturities, Gross Subtotal	36,578.5	36,430.9
Mortgage/Asset-Backed Securities	2,224.9	1,858.7
Fixed Maturity Securities, Amortized Cost	38,803.4	38,289.6
Fair Value Maturity Distribution [Abstract]
Total Fixed Maturity Securities	45,064.9	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	34.3	20.6
Over 1 year through 5 years	719.3	727.1
Over 5 years through 10 years	1,003.3	940.2
Over 10 years	4,422.4	2,486.2
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Gain, Gross Subtotal	6,179.3	4,174.1
Mortgage/Asset-Backed Securities	207.0	184.6
Total Fixed Maturity Securities	6,386.3	4,358.7
Fair Value Maturity Distribution [Abstract]
1 year or less	1,406.3	915.5
Over 1 year through 5 years	7,434.0	7,678.5
Over 5 years through 10 years	8,792.3	8,137.4
Over 10 years	22,490.6	18,441.5
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	40,123.2	35,172.9
Mortgage/Asset-Backed Securities	2,401.9	1,695.8
Total Fixed Maturity Securities	42,525.1	36,868.7
Fair Value of Fixed Maturity Securities in Unrealized Loss Position
Fair Value Maturity Distribution [Abstract]
1 year or less	0	9.0
Over 1 year through 5 years	147.1	146.2
Over 5 years through 10 years	1,663.0	2,199.6
Over 10 years	699.8	2,777.4
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	2,509.9	5,132.2
Mortgage/Asset-Backed Securities	29.9	343.5
Total Fixed Maturity Securities	2,539.8	5,475.7
Available-for-Sale Securities, Unrealized Loss Position, Gross Loss
1 year or less	0	0
Over 1 year through 5 years	9.4	0.6
Over 5 years through 10 years	80.9	95.8
Over 10 years	34.4	203.5
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Loss, Gross Subtotal	124.7	299.9
Mortgage/Asset-Backed Securities	0.1	4.0
Total Fixed Maturity Securities	$ 124.8	$ 303.9

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