Reserves for Losses and Loss Adjustment Expenses - Reserves for Losses and Loss Adjustment Expenses (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
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves beginning of the year				$ 2,562.9				$ 2,488.0	$ 2,562.9	$ 2,488.0	$ 2,180.2
Losses and LAE incurred during current calendar year, net of reinsurance:
Current accident year									1,082.6	1,058.8	1,058.4
Prior accident years									138.1	(18.0)	(8.2)
Losses and LAE incurred during calendar year, net of reinsurance	$ 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
Losses and LAE payments made during current calendar year, net of reinsurance:
Current accident year									224.3	273.3	289.6
Prior accident years									806.0	665.6	599.8
Losses and LAE payments made during current calendar year, net of reinsurance:									1,030.3	938.9	889.4
Change in participation interest									(14.4)	(25.5)	(23.2)
Foreign exchange adjustments									(16.2)	(44.9)	38.4
Net reserves - end of period	2,722.7				2,562.9				2,722.7	2,562.9	2,488.0
Reinsurance recoverables on unpaid losses and LAE, end of period	2,434.9				2,091.7				2,434.9	2,091.7	1,713.0
Reserves for losses and loss adjustment expenses	$ 5,157.6				$ 4,654.6				5,157.6	4,654.6	4,201.0
Maybrooke Holdings S A [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net Maybrooke reserves acquired									0.0	0.0	131.8
Ariscom Compagnia Di Assicurazioni S P A [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net Maybrooke reserves acquired									$ 0.0	$ 43.4	$ 0.0