Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Available-for-Sale Securities, Total Amortized Cost
1 year or less	$ 1,283.3	$ 1,372.0
Over 1 year through 5 years	6,784.1	6,871.2
Over 5 years through 10 years	10,002.5	9,532.9
Over 10 years	18,781.4	18,802.4
Available-for-sale Securities, Debt Maturities, Gross Subtotal	36,851.3	36,578.5
Mortgage/Asset-Backed Securities	2,174.6	2,224.9
Total Amortized Cost of Fixed Maturity Securities	39,025.9	38,803.4
Fair Value Maturity Distribution
Total Fixed Maturity Securities	45,852.7	45,064.9
Fair Value of Fixed Maturity Securities in Unrealized Gain Position
Available-for-Sale Securities, Unrealized Gain Position, Gross Gain
1 year or less	30.1	34.3
Over 1 year through 5 years	741.4	719.3
Over 5 years through 10 years	1,124.5	1,003.3
Over 10 years	4,779.9	4,422.4
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Gain, Gross Subtotal	6,675.9	6,179.3
Mortgage/Asset-Backed Securities	240.8	207.0
Total Fixed Maturity Securities	6,916.7	6,386.3
Fair Value Maturity Distribution
1 year or less	1,313.4	1,406.3
Over 1 year through 5 years	7,391.3	7,434.0
Over 5 years through 10 years	10,112.2	8,792.3
Over 10 years	23,123.4	22,490.6
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	41,940.3	40,123.2
Mortgage/Asset-Backed Securities	2,394.4	2,401.9
Total Fixed Maturity Securities	44,334.7	42,525.1
Fair Value of Fixed Maturity Securities in Unrealized Loss Position
Fair Value Maturity Distribution
1 year or less	0	0
Over 1 year through 5 years	119.4	147.1
Over 5 years through 10 years	968.0	1,663.0
Over 10 years	409.7	699.8
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	1,497.1	2,509.9
Mortgage/Asset-Backed Securities	20.9	29.9
Total Fixed Maturity Securities	1,518.0	2,539.8
Available-for-Sale Securities, Unrealized Loss Position, Gross Loss
1 year or less	0	0
Over 1 year through 5 years	14.8	9.4
Over 5 years through 10 years	46.8	80.9
Over 10 years	28.2	34.4
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Loss, Gross Subtotal	89.8	124.7
Mortgage/Asset-Backed Securities	0.1	0.1
Total Fixed Maturity Securities	$ 89.9	$ 124.8

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