Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Available-for-Sale Securities, Total Amortized Cost
1 year or less	$ 1,313.5	$ 1,112.2
Over 1 year through 5 years	6,304.0	6,514.0
Over 5 years through 10 years	11,073.3	10,519.3
Over 10 years	18,755.5	19,194.6
Available-for-sale Securities, Debt Maturities, Gross Subtotal	37,446.3	37,340.1
Mortgage/Asset-Backed Securities	2,178.8	2,318.6
Amortized Cost of Fixed Maturity Securities	39,625.1	39,658.7
Fair Value Maturity Distribution
Total Fixed Maturity Securities	46,315.5	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	26.9	20.6
Over 1 year through 5 years	634.1	554.4
Over 5 years through 10 years	1,269.9	746.3
Over 10 years	4,635.9	2,849.1
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Gain, Gross Subtotal	6,566.8	4,170.4
Mortgage/Asset-Backed Securities	207.7	167.6
Total Fixed Maturity Securities	6,774.5	4,338.0
Fair Value Maturity Distribution
1 year or less	1,333.6	1,098.8
Over 1 year through 5 years	6,693.0	6,649.5
Over 5 years through 10 years	11,663.0	7,124.4
Over 10 years	22,868.9	18,532.3
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	42,558.5	33,405.0
Mortgage/Asset-Backed Securities	2,376.0	1,968.2
Total Fixed Maturity Securities	44,934.5	35,373.2
Fair Value of Fixed Maturity Securities in Unrealized Loss Position
Fair Value Maturity Distribution
1 year or less	6.7	33.8
Over 1 year through 5 years	231.8	369.1
Over 5 years through 10 years	635.6	3,820.7
Over 10 years	496.7	3,244.3
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	1,370.8	7,467.9
Mortgage/Asset-Backed Securities	10.2	513.3
Total Fixed Maturity Securities	1,381.0	7,981.2
Available-for-Sale Securities, Unrealized Loss Position, Gross Loss
1 year or less	0.1	0.2
Over 1 year through 5 years	13.3	49.8
Over 5 years through 10 years	44.6	320.5
Over 10 years	25.8	267.1
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Loss, Gross Subtotal	83.8	637.6
Mortgage/Asset-Backed Securities	0.3	4.7
Total Fixed Maturity Securities	$ 84.1	$ 642.3