Losses and Loss Adjustment Expenses Incurred and Paid - Schedule of Incurred Losses and Loss Adjustment Expenses, Net of Reinsurance (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Claims Development [Line Items]
Incurred losses and loss adjustment expenses, net of reinsurance	$ 1,776,432
Accident Year 2008
Claims Development [Line Items]
Incurred losses and loss adjustment expenses, net of reinsurance	177,892	$ 177,226	$ 177,190	$ 176,947	$ 176,468	$ 175,849	$ 175,044	$ 174,312	$ 174,921	$ 187,583
Accident Year 2009
Claims Development [Line Items]
Incurred losses and loss adjustment expenses, net of reinsurance	142,703	142,705	142,546	142,772	142,819	141,936	140,501	140,539	$ 146,584
Accident Year 2010
Claims Development [Line Items]
Incurred losses and loss adjustment expenses, net of reinsurance	134,000	133,983	134,144	134,191	134,446	134,470	133,283	$ 135,458
Accident Year 2011
Claims Development [Line Items]
Incurred losses and loss adjustment expenses, net of reinsurance	125,703	125,670	125,810	126,373	126,272	126,886	$ 126,384
Accident Year 2012
Claims Development [Line Items]
Incurred losses and loss adjustment expenses, net of reinsurance	141,100	141,375	140,673	140,249	140,111	$ 144,231
Accident Year 2013
Claims Development [Line Items]
Incurred losses and loss adjustment expenses, net of reinsurance	144,450	144,076	142,053	140,775	$ 145,878
Accident Year 2014
Claims Development [Line Items]
Incurred losses and loss adjustment expenses, net of reinsurance	171,372	171,827	165,991	$ 166,179
Accident Year 2015
Claims Development [Line Items]
Incurred losses and loss adjustment expenses, net of reinsurance	240,388	240,419	$ 218,186
Accident Year 2016
Claims Development [Line Items]
Incurred losses and loss adjustment expenses, net of reinsurance	275,768	$ 278,366
Accident Year 2017
Claims Development [Line Items]
Incurred losses and loss adjustment expenses, net of reinsurance	$ 223,056