Variable Interest Entities - Schedule of Variable Interest Entities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets
Cash		$ 110,233
Total assets	$ 1,571,407	1,077,357
Liabilities
Total liabilities		913,127
Bank Holding Company Accounting [Member]
Assets
Cash	166
Net loans receivable	45,384
Accrued interest receivable	97
Loan collateral in process of foreclosure	17,865
Total assets	63,512
Liabilities
Accounts payable and accrued expenses	69
Accrued interest payable	2,364
Total liabilities	98,491
Investment Company Accounting [Member]
Assets
Cash		393
Net investments		96,688
Accrued interest receivable		170
Total assets		97,251
Liabilities
Accrued interest payable		1,849
Total liabilities		101,833
DZ Loan [Member] | Bank Holding Company Accounting [Member]
Liabilities
Loans	$ 96,058
DZ Loan [Member] | Investment Company Accounting [Member]
Liabilities
Loans		$ 99,984