Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Available-for-Sale Securities, Total Amortized Cost
1 year or less	$ 1,157.6	$ 1,112.2
Over 1 year through 5 years	6,657.1	6,514.0
Over 5 years through 10 years	10,669.6	10,519.3
Over 10 years	19,040.6	19,194.6
Available-for-sale Securities, Debt Maturities, Gross Subtotal	37,524.9	37,340.1
Mortgage/Asset-Backed Securities	2,279.8	2,318.6
Amortized Cost of Fixed Maturity Securities	39,804.7	39,658.7
Fair Value Maturity Distribution
Total Fixed Maturity Securities	44,699.2	43,354.4
Fair Value of Fixed Maturity Securities in Unrealized Gain Position
Available-for-Sale Securities, Unrealized Gain Position, Gross Gain
1 year or less	21.0	20.6
Over 1 year through 5 years	633.0	554.4
Over 5 years through 10 years	950.1	746.3
Over 10 years	3,513.9	2,849.1
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Gain, Gross Subtotal	5,118.0	4,170.4
Mortgage/Asset-Backed Securities	201.0	167.6
Total Fixed Maturity Securities	5,319.0	4,338.0
Fair Value Maturity Distribution
1 year or less	1,104.5	1,098.8
Over 1 year through 5 years	6,880.2	6,649.5
Over 5 years through 10 years	9,201.5	7,124.4
Over 10 years	20,484.6	18,532.3
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	37,670.8	33,405.0
Mortgage/Asset-Backed Securities	2,417.8	1,968.2
Total Fixed Maturity Securities	40,088.6	35,373.2
Fair Value of Fixed Maturity Securities in Unrealized Loss Position
Fair Value Maturity Distribution
1 year or less	73.7	33.8
Over 1 year through 5 years	346.8	369.1
Over 5 years through 10 years	2,217.3	3,820.7
Over 10 years	1,910.2	3,244.3
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	4,548.0	7,467.9
Mortgage/Asset-Backed Securities	62.6	513.3
Total Fixed Maturity Securities	4,610.6	7,981.2
Available-for-Sale Securities, Unrealized Loss Position, Gross Loss
1 year or less	0.4	0.2
Over 1 year through 5 years	63.1	49.8
Over 5 years through 10 years	200.9	320.5
Over 10 years	159.7	267.1
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Loss, Gross Subtotal	424.1	637.6
Mortgage/Asset-Backed Securities	0.4	4.7
Total Fixed Maturity Securities	$ 424.5	$ 642.3