Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) $ in Millions	Mar. 31, 2020 USD ($)
Amortized Cost
Due in one year or less	$ 306.6
Due after one year through five years	1,489.7
Due after five years through ten years	554.2
Thereafter	64.8
Structured securities	1,219.1
Total	3,634.4
Fair Value
Due in one year or less	301.6
Due after one year through five years	1,419.3
Due after five years through ten years	536.8
Thereafter	63.0
Structured securities	1,202.8
Total	$ 3,523.5

Amount of investment in debt security measured at amortized cost (held-to-maturity).

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