Investments Composition of Fixed Maturities by Maturity (Detail) $ in Millions	Sep. 30, 2017 USD ($)
Available for sale, cost
Less than one year	$ 4,088.1
One to five years	11,476.9
Five to ten years	2,873.9
Ten years or greater	144.2
Total	18,583.1
Available for sale, Fair value
Less than one year	4,105.7
One to five years	11,508.8
Five to ten years	2,901.3
Ten years or greater	144.2
Total	$ 18,660.0