Losses and Loss Adjustment Reserve - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2017 USD ($) Reinsurer	Dec. 31, 2016 USD ($)
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Ceded rate of premiums under insurance program	100.00%
Premiums written net	$ 462,710,000
Ceded unpaid loss	2,160,300,000
Prepaid reinsurance premiums	332,246,000	$ 308,661,000
Increase (Decrease) in Loss and Loss Adjustment Expense Reserve	0
Reserve for losses and loss adjustment expense	$ 2,160,300,000
Number of reinsurers | Reinsurer	2
Wright Flood
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Ceded rate of premiums under insurance program	100.00%
Expenses and allowance rate received in premiums	30.90%
Premiums written net	$ 461,500,000
Wright Flood | Quota Share Agreement
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Effective cedes rate under quota share agreement	100.00%
Commission rate, percent of ceded written premiums	30.50%
Ceded amount	$ 1,200,000