Reinsurance - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Insurance [Abstract]
Reinsurance recoverables, allowance for doubtful accounts	$ 1.8				$ 2.1				$ 1.8	$ 2.1
Amount of collateral under reinsurance agreement	944.0				907.7				944.0	907.7
Paid loss recoverables in insurance recoverables	596.6				380.3				596.6	380.3
Losses and loss adjustment expenses	$ 280.6	$ 277.5	$ 245.5	$ 237.2	$ 270.7	$ 326.4	$ 230.6	$ 222.5	1,040.8	1,050.2	$ 810.1
Net of amounts ceded to reinsurers									$ 888.9	$ 992.6	$ 419.3