Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) $ in Millions	Jun. 30, 2020 USD ($)
Amortized Cost
Due in one year or less	$ 316.3
Due after one year through five years	1,540.7
Due after five years through ten years	579.8
Thereafter	71.2
Structured securities	1,180.6
Total	3,688.6
Fair Value
Due in one year or less	316.8
Due after one year through five years	1,523.5
Due after five years through ten years	592.4
Thereafter	71.5
Structured securities	1,203.2
Total	$ 3,707.4