Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Available-for-Sale Securities, Total Amortized Cost
1 year or less	$ 1,112.2	$ 1,372.0
Over 1 year through 5 years	6,514.0	6,871.2
Over 5 years through 10 years	10,519.3	9,532.9
Over 10 years	19,194.6	18,802.4
Available-for-sale Securities, Debt Maturities, Gross Subtotal	37,340.1	36,578.5
Mortgage/Asset-Backed Securities	2,318.6	2,224.9
Amortized Cost of Fixed Maturity Securities	39,658.7	38,803.4
Fair Value Maturity Distribution [Abstract]
Total Fixed Maturity Securities	43,354.4	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	20.6	34.3
Over 1 year through 5 years	554.4	719.3
Over 5 years through 10 years	746.3	1,003.3
Over 10 years	2,849.1	4,422.4
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Gain, Gross Subtotal	4,170.4	6,179.3
Mortgage/Asset-Backed Securities	167.6	207.0
Total Fixed Maturity Securities	4,338.0	6,386.3
Fair Value Maturity Distribution [Abstract]
1 year or less	1,098.8	1,406.3
Over 1 year through 5 years	6,649.5	7,434.0
Over 5 years through 10 years	7,124.4	8,792.3
Over 10 years	18,532.3	22,490.6
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	33,405.0	40,123.2
Mortgage/Asset-Backed Securities	1,968.2	2,401.9
Total Fixed Maturity Securities	35,373.2	42,525.1
Fair Value of Fixed Maturity Securities in Unrealized Loss Position
Fair Value Maturity Distribution [Abstract]
1 year or less	33.8	0.0
Over 1 year through 5 years	369.1	147.1
Over 5 years through 10 years	3,820.7	1,663.0
Over 10 years	3,244.3	699.8
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	7,467.9	2,509.9
Mortgage/Asset-Backed Securities	513.3	29.9
Total Fixed Maturity Securities	7,981.2	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	49.8	9.4
Over 5 years through 10 years	320.5	80.9
Over 10 years	267.1	34.4
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Loss, Gross Subtotal	637.6	124.7
Mortgage/Asset-Backed Securities	4.7	0.1
Total Fixed Maturity Securities	$ 642.3	$ 124.8