Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) $ in Millions	Jun. 30, 2018 USD ($)
Amortized Cost
Due in one year or less	$ 208.9
Due after one year through five years	1,482.3
Due after five years through ten years	597.2
Thereafter	149.7
Structured securities	900.4
Total	3,338.5
Fair Value
Due in one year or less	208.6
Due after one year through five years	1,465.7
Due after five years through ten years	588.0
Thereafter	149.1
Structured securities	890.0
Total	$ 3,301.4