Accounts Payable and Other Liabilities - Schedule of Other Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Other Liabilities Disclosure [Abstract]
Deferred ceding commission	$ 43,291	$ 51,109
Outstanding claim checks	2,309	79,666
Accounts payable and other payables	13,755	17,948
Accrued interest and issuance costs	757	3,117
Accrued dividends	1,594
Other liabilities	3,096	5,088
Commission payables	12,958	12,609
Total other liabilities	$ 77,760	$ 169,537