Loss Reserves - Additional Information (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($) Segment	Sep. 30, 2016 USD ($)
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Increase in incurred losses and LAE	$ 129,900
Provision for Losses and LAE for Claims Occurring in the Current Year	578,272	$ 504,599
Net prior accident year reserve strengthening	$ 44,609	$ (11,644)
Number of reportable segments | Segment	3
Increase in losses and loss adjustment expense payments	$ 37,700
Maria
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	26,400
Irma
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	25,800
Harvey
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	17,100
Puebla, Mexico Earthquake
Causes Of Increase Decrease In Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	$ 3,500