Investments Composition of Fixed Maturities by Maturity (Detail) - Total fixed maturities $ in Millions	Mar. 31, 2021 USD ($)
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Amortized Cost [Abstract]
Less than one year	$ 4,918.5
One to five years	24,370.8
Five to ten years	9,214.5
Ten years or greater	60.2
Cost	38,564.0
Fair Value
Less than one year	4,940.6
One to five years	24,814.6
Five to ten years	9,277.3
Ten years or greater	59.3
Fair Value	$ 39,091.8

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 through tenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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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 second through fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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