Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Available-for-Sale Securities, Total Amortized Cost
1 year or less	$ 1,694.9	$ 1,625.1
Over 1 year through 5 years	5,665.0	5,579.9
Over 5 years through 10 years	12,273.8	12,091.1
Over 10 years	18,741.8	18,611.2
Available-for-sale Securities, Debt Maturities, Gross Subtotal	38,375.5	37,907.3
Mortgage/Asset-Backed Securities	1,776.4	1,873.2
Amortized Cost of Fixed Maturity Securities	40,151.9	39,780.5
Available-for-Sale Securities, Unrealized Gain Position, Gross Gain
1 year or less	23.4	30.9
Over 1 year through 5 years	372.0	453.6
Over 5 years through 10 years	903.8	1,169.8
Over 10 years	3,261.0	4,041.4
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Gain, Gross Subtotal	4,560.2	5,695.7
Mortgage/Asset-Backed Securities	81.1	105.1
Total Fixed Maturity Securities	4,641.3	5,800.8
Fair Value Maturity Distribution
Fixed Maturity Securities	44,500.1
Available-for-Sale Securities, Unrealized Loss Position, Gross Loss
1 year or less	4.1	3.1
Over 1 year through 5 years	22.3	18.5
Over 5 years through 10 years	138.1	53.2
Over 10 years	113.9	44.0
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Loss, Gross Subtotal	278.4	118.8
Mortgage/Asset-Backed Securities	14.7	4.7
Total Fixed Maturity Securities	293.1	123.5
Fair Value of Fixed Maturity Securities in Unrealized Gain Position
Fair Value Maturity Distribution
1 year or less	1,645.0	1,638.8
Over 1 year through 5 years	5,680.4	5,810.9
Over 5 years through 10 years	9,124.4	11,916.5
Over 10 years	19,099.4	21,333.1
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	35,549.2	40,699.3
Mortgage/Asset-Backed Securities	1,362.4	1,576.4
Fixed Maturity Securities	36,911.6	42,275.7
Fair Value of Fixed Maturity Securities in Unrealized Loss Position
Fair Value Maturity Distribution
1 year or less	69.2	14.1
Over 1 year through 5 years	334.3	204.1
Over 5 years through 10 years	3,915.1	1,291.2
Over 10 years	2,789.5	1,275.5
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	7,108.1	2,784.9
Mortgage/Asset-Backed Securities	480.4	397.2
Fixed Maturity Securities	$ 7,588.5	$ 3,182.1