Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Available-for-Sale Securities, Total Amortized Cost
1 year or less	$ 1,204.7	$ 1,372.0
Over 1 year through 5 years	6,755.9	6,871.2
Over 5 years through 10 years	10,103.6	9,532.9
Over 10 years	19,441.6	18,802.4
Available-for-sale Securities, Debt Maturities, Gross Subtotal	37,505.8	36,578.5
Mortgage/Asset-Backed Securities	2,232.3	2,224.9
Amortized Cost of Fixed Maturity Securities	39,738.1	38,803.4
Fair Value Maturity Distribution
Total Fixed Maturity Securities	44,401.5	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	23.5	34.3
Over 1 year through 5 years	658.3	719.3
Over 5 years through 10 years	825.9	1,003.3
Over 10 years	3,349.6	4,422.4
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Gain, Gross Subtotal	4,857.3	6,179.3
Mortgage/Asset-Backed Securities	198.4	207.0
Total Fixed Maturity Securities	5,055.7	6,386.3
Fair Value Maturity Distribution
1 year or less	1,211.1	1,406.3
Over 1 year through 5 years	7,135.7	7,434.0
Over 5 years through 10 years	7,511.3	8,792.3
Over 10 years	19,958.9	22,490.6
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	35,817.0	40,123.2
Mortgage/Asset-Backed Securities	2,316.4	2,401.9
Total Fixed Maturity Securities	38,133.4	42,525.1
Fair Value of Fixed Maturity Securities in Unrealized Loss Position
Fair Value Maturity Distribution
1 year or less	16.9	0.0
Over 1 year through 5 years	263.3	147.1
Over 5 years through 10 years	3,213.5	1,663.0
Over 10 years	2,661.1	699.8
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	6,154.8	2,509.9
Mortgage/Asset-Backed Securities	113.3	29.9
Total Fixed Maturity Securities	6,268.1	2,539.8
Available-for-Sale Securities, Unrealized Loss Position, Gross Loss
1 year or less	0.2	0.0
Over 1 year through 5 years	15.2	9.4
Over 5 years through 10 years	204.7	80.9
Over 10 years	171.2	34.4
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Loss, Gross Subtotal	391.3	124.7
Mortgage/Asset-Backed Securities	1.0	0.1
Total Fixed Maturity Securities	$ 392.3	$ 124.8