Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Available-for-Sale Securities, Total Amortized Cost
1 year or less	$ 1,625.1	$ 1,338.8
Over 1 year through 5 years	5,579.9	6,231.0
Over 5 years through 10 years	12,091.1	10,991.6
Over 10 years	18,611.2	18,886.4
Available-for-sale Securities, Debt Maturities, Gross Subtotal	37,907.3	37,447.8
Mortgage/Asset-Backed Securities	1,873.2	2,104.9
Amortized Cost of Fixed Maturity Securities	39,780.5	39,552.7
Fair Value Maturity Distribution [Abstract]
Total Fixed Maturity Securities	45,457.8	44,217.3
Fair Value of Fixed Maturity Securities in Unrealized Gain Position
Available-for-Sale Securities, Unrealized Gain Position, Gross Gain
1 year or less	30.9	28.6
Over 1 year through 5 years	453.6	553.5
Over 5 years through 10 years	1,169.8	843.8
Over 10 years	4,041.4	3,322.6
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Gain, Gross Subtotal	5,695.7	4,748.5
Mortgage/Asset-Backed Securities	105.1	134.7
Total Fixed Maturity Securities	5,800.8	4,883.2
Fair Value Maturity Distribution [Abstract]
1 year or less	1,638.8	1,355.6
Over 1 year through 5 years	5,810.9	6,605.6
Over 5 years through 10 years	11,916.5	9,336.2
Over 10 years	21,333.1	19,734.3
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	40,699.3	37,031.7
Mortgage/Asset-Backed Securities	1,576.4	1,716.4
Total Fixed Maturity Securities	42,275.7	38,748.1
Fair Value of Fixed Maturity Securities in Unrealized Loss Position
Fair Value Maturity Distribution [Abstract]
1 year or less	14.1	11.8
Over 1 year through 5 years	204.1	170.7
Over 5 years through 10 years	1,291.2	2,416.4
Over 10 years	1,275.5	2,356.3
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	2,784.9	4,955.2
Mortgage/Asset-Backed Securities	397.2	514.0
Total Fixed Maturity Securities	3,182.1	5,469.2
Available-for-Sale Securities, Unrealized Loss Position, Gross Loss
1 year or less	3.1	0.0
Over 1 year through 5 years	18.5	8.2
Over 5 years through 10 years	53.2	82.8
Over 10 years	44.0	118.4
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Loss, Gross Subtotal	118.8	209.4
Mortgage/Asset-Backed Securities	4.7	9.2
Total Fixed Maturity Securities	$ 123.5	$ 218.6