Investments Composition of Fixed Maturities by Maturity (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Schedule of Debt Securities by Contractual Maturity [Line Items]
Fixed maturities (amortized cost: $24,061.8, $18,311.7, and $20,209.9)	$ 27,020.4	$ 23,071.1	$ 22,117.8
Total fixed maturities
Schedule of Debt Securities by Contractual Maturity [Line Items]
Less than one year	3,805.2
One to five years	15,190.7
Five to ten years	4,939.7
Ten years or greater	126.2
Cost	24,061.8
Less than one year	3,809.2
One to five years	14,999.1
Five to ten years	4,855.4
Ten years or greater	125.5
Fixed maturities (amortized cost: $24,061.8, $18,311.7, and $20,209.9)	$ 23,789.2