Variable Interest Entities - Summary of Reconciliation of Carrying Value of Residential Mortgage Loans (Detail) - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Variable Interest Entity [Line Items]
Balance at beginning of period	[1]	$ 549,016
Deductions during period:
Balance at end of period	[1]	535,077		$ 549,016
Residential Mortgage Backed Securities
Variable Interest Entity [Line Items]
Balance at beginning of period		11,660
Loan acquisitions		118,765		11,660
Deductions during period:
Collections of principal		(816)
Amortization of premium		(26)
Balance at end of period		129,583		11,660
Variable Interest Entities Primary Beneficiary | Residential Mortgage Backed Securities
Variable Interest Entity [Line Items]
Balance at beginning of period		549,016	$ 639,351	639,351
Deductions during period:
Collections of principal		(13,520)	(27,813)	(88,338)
Amortization of premium		(430)	(549)	(2,014)
Charge offs, net		11	17	17
Balance at end of period		$ 535,077	$ 611,006	$ 549,016

[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 March 31, 2019 and December 31, 2018, total assets of the consolidated VIEs were $537 million and $551 million (including accrued interest receivable of $1.8 million and $1.8 million), respectively (which is recorded above in the line item entitled “Interest and dividends receivable”), and total liabilities were $527 million and $541 million (including accrued interest payable of $1.7 million and $1.7 million), respectively (which is recorded in the line item above entitled “Accrued interest payable”). Please refer to Note 5, “Variable Interest Entities,” for further discussion.