Reinsurance - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Insurance [Abstract]
Reinsurance recoverables, allowance for doubtful accounts	$ 1.1				$ 1.8				$ 1.1	$ 1.8
Amount of collateral under reinsurance agreement	1,184.7				944.0				1,184.7	944.0
Paid loss recoverables in insurance recoverables	669.7				596.6				669.7	596.6
Losses and loss adjustment expenses	$ 359.2	$ 338.8	$ 284.8	$ 237.9	$ 280.6	$ 277.5	$ 245.5	$ 237.2	1,220.7	1,040.8	$ 1,050.2
Net of amounts ceded to reinsurers									$ 1,031.1	$ 888.9	$ 992.6