Reinsurance - Schedule of Reinsurance Premium (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]
Direct written premiums									$ 2,293.8	$ 2,029.2	$ 1,792.5
Reinsurance ceded to other companies									(1,189.7)	(1,043.7)	(724.6)
Reinsurance assumed from other companies									661.4	668.0	372.3
Net written premiums									1,765.5	1,653.5	1,440.2
Direct earned premiums									2,201.9	1,912.2	1,722.8
Reinsurance ceded to other companies									(1,137.9)	(1,033.6)	(675.8)
Reinsurance assumed from other companies									667.7	693.7	363.8
Net earned premiums	$ 452.4	$ 446.9	$ 417.7	$ 414.7	$ 404.5	$ 389.3	$ 399.1	$ 379.4	$ 1,731.7	$ 1,572.3	$ 1,410.8
Percentage of reinsurance assumed to net earned premiums									38.60%	44.10%	25.80%