Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Amortized Cost
Due in one year or less	$ 281.7
Due after one year through five years	1,692.0
Due after five years through ten years	710.0
Thereafter	112.6
Structured securities	4,203.2
Amortized Cost	4,203.2	$ 3,981.1
Fair Value
Due in one year or less	284.6
Due after one year through five years	1,704.8
Due after five years through ten years	711.3
Thereafter	111.6
Structured securities		4,223.3
Fair Value	4,223.3	$ 4,107.1
Structured Securities
Amortized Cost
Structured securities	1,406.9
Fair Value
Structured securities	$ 1,411.0

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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