Other Assets - Summary of Other Assets (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Receivables from sales and maturities of investment securities	$ 15,530	$ 255
Benefits receivable related to retrospective reinsurance contracts	2,579	2,393
Prepaid expenses	1,902	1,741
Lease acquisition costs, net	670	723
Other	2,812	4,629
Total other assets	$ 23,493	$ 9,741