Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Available-for-Sale Securities, Total Amortized Cost
1 year or less	$ 1,238.4	$ 1,372.0
Over 1 year through 5 years	6,882.6	6,871.2
Over 5 years through 10 years	10,057.0	9,532.9
Over 10 years	19,102.1	18,802.4
Available-for-sale Securities, Debt Maturities, Gross Subtotal	37,280.1	36,578.5
Mortgage/Asset-Backed Securities	2,389.1	2,224.9
Amortized Cost of Fixed Maturity Securities	39,669.2	38,803.4
Fair Value Maturity Distribution
Total Fixed Maturity Securities	44,570.4	45,064.9
Fair Value of Fixed Maturity Securities in Unrealized Gain Position
Available-for-Sale Securities, Unrealized Gain Position, Gross Gain
1 year or less	26.7	34.3
Over 1 year through 5 years	712.8	719.3
Over 5 years through 10 years	852.2	1,003.3
Over 10 years	3,329.5	4,422.4
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Gain, Gross Subtotal	4,921.2	6,179.3
Mortgage/Asset-Backed Securities	195.9	207.0
Total Fixed Maturity Securities	5,117.1	6,386.3
Fair Value Maturity Distribution
1 year or less	1,250.1	1,406.3
Over 1 year through 5 years	7,476.1	7,434.0
Over 5 years through 10 years	7,987.7	8,792.3
Over 10 years	19,886.9	22,490.6
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	36,600.8	40,123.2
Mortgage/Asset-Backed Securities	2,214.1	2,401.9
Total Fixed Maturity Securities	38,814.9	42,525.1
Fair Value of Fixed Maturity Securities in Unrealized Loss Position
Fair Value Maturity Distribution
1 year or less	14.9	0.0
Over 1 year through 5 years	114.3	147.1
Over 5 years through 10 years	2,827.8	1,663.0
Over 10 years	2,433.1	699.8
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	5,390.1	2,509.9
Mortgage/Asset-Backed Securities	365.4	29.9
Total Fixed Maturity Securities	5,755.5	2,539.8
Available-for-Sale Securities, Unrealized Loss Position, Gross Loss
1 year or less	0.1	0.0
Over 1 year through 5 years	5.0	9.4
Over 5 years through 10 years	93.7	80.9
Over 10 years	111.6	34.4
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Loss, Gross Subtotal	210.4	124.7
Mortgage/Asset-Backed Securities	5.5	0.1
Total Fixed Maturity Securities	$ 215.9	$ 124.8