Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Available-for-Sale Securities, Total Amortized Cost
1 year or less	$ 1,338.2	$ 1,112.2
Over 1 year through 5 years	6,321.2	6,514.0
Over 5 years through 10 years	10,941.7	10,519.3
Over 10 years	18,700.3	19,194.6
Available-for-sale Securities, Debt Maturities, Gross Subtotal	37,301.4	37,340.1
Mortgage/Asset-Backed Securities	2,261.1	2,318.6
Amortized Cost of Fixed Maturity Securities	39,562.5	39,658.7
Fair Value Maturity Distribution
Total Fixed Maturity Securities	45,862.0	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	27.0	20.6
Over 1 year through 5 years	644.5	554.4
Over 5 years through 10 years	1,169.6	746.3
Over 10 years	4,409.6	2,849.1
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Gain, Gross Subtotal	6,250.7	4,170.4
Mortgage/Asset-Backed Securities	214.4	167.6
Total Fixed Maturity Securities	6,465.1	4,338.0
Fair Value Maturity Distribution
1 year or less	1,282.1	1,098.8
Over 1 year through 5 years	6,614.9	6,649.5
Over 5 years through 10 years	10,747.3	7,124.4
Over 10 years	22,502.7	18,532.3
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	41,147.0	33,405.0
Mortgage/Asset-Backed Securities	2,453.5	1,968.2
Total Fixed Maturity Securities	43,600.5	35,373.2
Fair Value of Fixed Maturity Securities in Unrealized Loss Position
Fair Value Maturity Distribution
1 year or less	82.8	33.8
Over 1 year through 5 years	323.6	369.1
Over 5 years through 10 years	1,277.3	3,820.7
Over 10 years	556.1	3,244.3
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	2,239.8	7,467.9
Mortgage/Asset-Backed Securities	21.7	513.3
Total Fixed Maturity Securities	2,261.5	7,981.2
Available-for-Sale Securities, Unrealized Loss Position, Gross Loss
1 year or less	0.3	0.2
Over 1 year through 5 years	27.2	49.8
Over 5 years through 10 years	86.7	320.5
Over 10 years	51.1	267.1
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Loss, Gross Subtotal	165.3	637.6
Mortgage/Asset-Backed Securities	0.3	4.7
Total Fixed Maturity Securities	$ 165.6	$ 642.3