Variable Interest Entities - Summary of Assets and Liabilities of Variable Interest Entities (Detail) - USD ($) $ in Thousands		Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Residential mortgage loans held-for-investment	[1]	$ 585,020		$ 639,351
Accrued interest receivable		17,272		18,091
Accrued interest payable		21,881		15,835
Asset-backed securities issued by securitization trusts	[1]	575,653		629,984
Variable Interest Entities Primary Beneficiary
Variable Interest Entity [Line Items]
Accrued interest receivable		1,900		2,100
Accrued interest payable		1,900		2,000
Asset-backed securities issued by securitization trusts		575,700		630,000
Variable Interest Entities Primary Beneficiary | Residential Mortgage Backed Securities
Variable Interest Entity [Line Items]
Residential mortgage loans held-for-investment		585,020	$ 611,006	639,351	$ 744,462
Accrued interest receivable		1,934		2,093
Total assets		586,954		641,444
Accrued interest payable		1,887		2,047
Asset-backed securities issued by securitization trusts		575,653		629,984
Total liabilities		$ 577,540		$ 632,031

[1] The consolidated balance sheets include assets of consolidated variable interest entities, or VIEs, that can only be used to settle obligations and liabilities of the VIEs for which creditors do not have recourse to the Company. At June 30, 2018 and December 31, 2017, total assets of the consolidated VIEs were $587 million and $641 million (including accrued interest receivable of $1.9 million and $2.1 million), respectively (which is recorded above in the line item entitled “Interest and dividends receivable”), and total liabilities were $578 million and $632 million (including accrued interest payable of $1.9 million and $2.0 million), respectively (which is recorded in the line item above entitled “Accrued interest payable”). Please refer to Note 4, “Variable Interest Entities,” for further discussion.