Reinsurance - Schedule of Reinsurance Premium (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]
Direct written premiums									$ 2,509.5	$ 2,293.8	$ 2,029.2
Reinsurance ceded to other companies									(1,374.8)	(1,189.7)	(1,043.7)
Reinsurance assumed from other companies									619.7	661.4	668.0
Net written premiums									1,754.4	1,765.5	1,653.5
Direct earned premiums									2,411.5	2,201.9	1,912.2
Reinsurance ceded to other companies									(1,286.0)	(1,137.9)	(1,033.6)
Reinsurance assumed from other companies									604.0	667.7	693.7
Net earned premiums	$ 425.8	$ 451.5	$ 431.7	$ 420.5	$ 452.4	$ 446.9	$ 417.7	$ 414.7	$ 1,729.5	$ 1,731.7	$ 1,572.3
Percentage of reinsurance assumed to net earned premiums									34.90%	38.60%	44.10%