Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) $ in Millions	Dec. 31, 2019 USD ($)
Amortized Cost
Due in one year or less	$ 294.2
Due after one year through five years	1,495.0
Due after five years through ten years	653.2
Thereafter	79.1
Structured securities	1,083.5
Total	3,605.0
Fair Value
Due in one year or less	294.8
Due after one year through five years	1,491.0
Due after five years through ten years	666.6
Thereafter	83.9
Structured securities	1,097.2
Total	$ 3,633.5