Investments Composition of Fixed Maturities by Maturity (Detail) - Fixed maturities $ in Millions	Dec. 31, 2019 USD ($)
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Amortized Cost [Abstract]
Less than one year	$ 6,700.1
One to five years	16,898.3
Five to ten years	9,010.8
Ten years or greater	33.9
Cost	32,643.1
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value [Abstract]
Less than one year	6,712.6
One to five years	17,158.4
Five to ten years	9,203.9
Ten years or greater	35.4
Debt Securities, Available-for-Sale	$ 33,110.3

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth fiscal year through tenth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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