Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Amortized Cost
Due in one year or less	$ 324.9
Due after one year through five years	1,743.6
Due after five years through ten years	607.8
Thereafter	82.2
Structured securities	1,222.6
Total	3,981.1
Fair Value
Due in one year or less	329.7
Due after one year through five years	1,791.9
Due after five years through ten years	639.3
Thereafter	85.9
Structured securities	$ 1,260.3
Total		$ 4,107.1