Underwriting, Acquisition and Insurance Expenses (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Underwriting Acquisition And Insurance Expenses [Abstract]
Commissions	$ 65.3	$ 63.3
General expenses	98.9	88.6
Premium taxes, boards and bureaus	8.0	8.3
Underwriting, acquisition and insurance expenses, total	172.2	160.2
Net deferral of policy acquisition costs	(4.2)	0.0
Total underwriting, acquisition and insurance expenses	168.0	160.2
Other corporate expenses	$ 3.3	$ 0.5	$ 3.3