Losses and Loss Adjustment Reserve - Effects of Reinsurance on Premiums Written and Earned (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Written
Direct premiums	$ 604,623	$ 591,142
Assumed premiums	0	0
Ceded premiums	604,610	591,124
Net premiums	13	18
Earned
Direct premiums	592,267	592,123
Assumed premiums	0	0
Ceded premiums	592,254	592,105
Net premiums	$ 13	$ 18