Insurance Activity - Schedule of Premiums Assumed and Earned (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Insurance [Abstract]
Premiums assumed	$ 989	$ 2,656	$ 4,788
Change in unearned premiums	231	491	333
Premiums earned	$ 1,220	$ 3,147	$ 5,121