Losses and Loss Adjustment Expenses Incurred and Paid - Schedule of Frequency of Claims (Detail) $ in Thousands	Dec. 31, 2017 USD ($) Claim	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	$ 1,776,432
Accident Year 2008
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	$ 177,892	$ 177,226	$ 177,190	$ 176,947	$ 176,468	$ 175,849	$ 175,044	$ 174,312	$ 174,921	$ 187,583
Cumulative number of reported claims | Claim	100,189
Accident Year 2009
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	$ 142,703	142,705	142,546	142,772	142,819	141,936	140,501	140,539	$ 146,584
Cumulative number of reported claims | Claim	78,607
Accident Year 2010
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	$ 134,000	133,983	134,144	134,191	134,446	134,470	133,283	$ 135,458
Total of incurred but not reported liabilities plus expected development on reported claims	$ 17
Cumulative number of reported claims | Claim	71,138
Accident Year 2011
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	$ 125,703	125,670	125,810	126,373	126,272	126,886	$ 126,384
Total of incurred but not reported liabilities plus expected development on reported claims	$ 138
Cumulative number of reported claims | Claim	66,310
Accident Year 2012
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	$ 141,100	141,375	140,673	140,249	140,111	$ 144,231
Total of incurred but not reported liabilities plus expected development on reported claims	$ 366
Cumulative number of reported claims | Claim	73,546
Accident Year 2013
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	$ 144,450	144,076	142,053	140,775	$ 145,878
Total of incurred but not reported liabilities plus expected development on reported claims	$ 741
Cumulative number of reported claims | Claim	74,057
Accident Year 2014
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	$ 171,372	171,827	165,991	$ 166,179
Total of incurred but not reported liabilities plus expected development on reported claims	$ 2,449
Cumulative number of reported claims | Claim	85,334
Accident Year 2015
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	$ 240,388	240,419	$ 218,186
Total of incurred but not reported liabilities plus expected development on reported claims	$ 9,183
Cumulative number of reported claims | Claim	109,847
Accident Year 2016
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	$ 275,768	$ 278,366
Total of incurred but not reported liabilities plus expected development on reported claims	$ 27,195
Cumulative number of reported claims | Claim	117,923
Accident Year 2017
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	$ 223,056
Total of incurred but not reported liabilities plus expected development on reported claims	$ 63,131
Cumulative number of reported claims | Claim	85,508