Variable Interest Entities - Carrying Value for Residential Mortgage Loans (Detail) - USD ($) $ in Thousands		Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Variable Interest Entity [Line Items]
Carrying value	[1]	$ 585,020		$ 639,351
Variable Interest Entities Primary Beneficiary | Residential Mortgage Backed Securities
Variable Interest Entity [Line Items]
Principal balance		580,938		634,219
Unamortized premium, net of discount		4,268		5,335
Allowance for loan losses		(186)	$ (186)	(203)		$ (203)
Carrying value		$ 585,020	$ 611,006	$ 639,351	$ 744,462

[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.