Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Available-for-Sale Securities, Total Amortized Cost
1 year or less	$ 1,338.8	$ 1,112.2
Over 1 year through 5 years	6,231.0	6,514.0
Over 5 years through 10 years	10,991.6	10,519.3
Over 10 years	18,886.4	19,194.6
Available-for-sale Securities, Debt Maturities, Gross Subtotal	37,447.8	37,340.1
Mortgage/Asset-Backed Securities	2,104.9	2,318.6
Amortized Cost of Fixed Maturity Securities	39,552.7	39,658.7
Fair Value Maturity Distribution [Abstract]
Total Fixed Maturity Securities	44,217.3	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	28.6	20.6
Over 1 year through 5 years	553.5	554.4
Over 5 years through 10 years	843.8	746.3
Over 10 years	3,322.6	2,849.1
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Gain, Gross Subtotal	4,748.5	4,170.4
Mortgage/Asset-Backed Securities	134.7	167.6
Total Fixed Maturity Securities	4,883.2	4,338.0
Fair Value Maturity Distribution [Abstract]
1 year or less	1,355.6	1,098.8
Over 1 year through 5 years	6,605.6	6,649.5
Over 5 years through 10 years	9,336.2	7,124.4
Over 10 years	19,734.3	18,532.3
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	37,031.7	33,405.0
Mortgage/Asset-Backed Securities	1,716.4	1,968.2
Total Fixed Maturity Securities	38,748.1	35,373.2
Fair Value of Fixed Maturity Securities in Unrealized Loss Position
Fair Value Maturity Distribution [Abstract]
1 year or less	11.8	33.8
Over 1 year through 5 years	170.7	369.1
Over 5 years through 10 years	2,416.4	3,820.7
Over 10 years	2,356.3	3,244.3
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	4,955.2	7,467.9
Mortgage/Asset-Backed Securities	514.0	513.3
Total Fixed Maturity Securities	5,469.2	7,981.2
Available-for-Sale Securities, Unrealized Loss Position, Gross Loss
1 year or less	0.0	0.2
Over 1 year through 5 years	8.2	49.8
Over 5 years through 10 years	82.8	320.5
Over 10 years	118.4	267.1
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Loss, Gross Subtotal	209.4	637.6
Mortgage/Asset-Backed Securities	9.2	4.7
Total Fixed Maturity Securities	$ 218.6	$ 642.3