Variable Interest Entities - Additional Information (Detail)				1 Months Ended	2 Months Ended	3 Months Ended							6 Months Ended	11 Months Ended	12 Months Ended	24 Months Ended
	Nov. 02, 2018 USD ($) Trust	Oct. 10, 2017 USD ($) Trust	Oct. 09, 2017 USD ($) Trust	Feb. 09, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) Trust	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2017 USD ($) Trust	Dec. 31, 2018 USD ($) Trust	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) Trust
Variable Interest Entity [Line Items]
Number of securitization trusts related to Residual Trusts | Trust						4								4		4
Proceeds from the sale of residual interests in Residual Trusts				$ 0										$ 29,924,000	$ 0
Gain on sale of Residual Trusts				0										29,214,000	0
Gain on sale of Residual Trusts related to reversal of deferred interest income				3,740,000										11,083,000	2,008,000
Loss on sale of Residual trusts within other gains (losses)				0										$ (8,515,000)	(7,219,000)
Number of securitization trusts related to Non Residual Trusts | Trust						3								3		3
Original number of Non-Residual Trusts consolidated | Trust			10
Number of trusts for which the mandatory clean-up call obligation was fulfilled | Trust																7
Gain recognized				467,439,000	$ 484,000	$ (11,516,000)	$ 10,463,000	$ 12,987,000	$ (33,622,000)	$ 2,824,000	$ (8,807,000)	$ 72,810,000		$ 12,418,000	33,205,000
Amount of inducement fee expensed				$ 500,000					7,200,000					17,800,000
Iinducement fee balance						$ 11,022,000			29,256,000					$ 11,022,000	$ 29,256,000	$ 11,022,000
Mandatory Clean-up Call Obligation [Member]
Variable Interest Entity [Line Items]
Number of trusts for which the mandatory clean-up call obligation was fulfilled | Trust													2
Mandatory cleanup calls required when loan pool falls to percent of original principal balance						10.00%								10.00%		10.00%
Payments for mandatory clean-up call obligations													$ 28,400,000
Residential loans acquired													25,100,000
REO acquired													$ 100,000
Obligation To Reimburse Third Party As Credit Enhancement To Trusts [Member]
Variable Interest Entity [Line Items]
Aggregate number of securitization trusts with reimbursement obligations | Trust		11	11
Reimbursement obligation on LOC, if drawn			$ 165,000,000
Obligation to reimburse amounts drawn on LOCs in excess of threshold						$ 17,000,000								$ 17,000,000		$ 17,000,000
Variable Interest Entity, Primary Beneficiary [Member] | Non-Residual Trusts [Member]
Variable Interest Entity [Line Items]
Number of securitization trusts with reimbursement obligations | Trust		2	7
Variable Interest Entity, Not Primary Beneficiary [Member] | Servicing Arrangements With Letter Of Credit Reimbursement Obligation [Member]
Variable Interest Entity [Line Items]
Number of securitization trusts with reimbursement obligations | Trust			4
Sale Of Residual Interests In Residual Trusts [Member]
Variable Interest Entity [Line Items]
Number of Residual Trusts sold | Trust	4
Proceeds from the sale of residual interests in Residual Trusts	$ 41,000,000
Assets of Residual Trusts deconsolidated	414,200,000
Liabilities of Residual Trusts deconsolidated	402,600,000
Servicing right liability recorded	200,000
Gain on sale of Residual Trusts	29,200,000
Gain on sale of Residual Trusts related to reversal of deferred interest income	1,000,000
Gain on sale of Residual Trusts related to the reversal of deferred income	37,300,000
Loss on sale of Residual trusts within other gains (losses)	$ 9,100,000
Clean-Up Call Agreement Transactions [Member]
Variable Interest Entity [Line Items]
Number of securitization trusts related to Non Residual Trusts | Trust		8
Number of trusts for which the mandatory clean-up call obligation was fulfilled | Trust						1								4
Payment of inducement fee		$ 36,500,000
Right to receive reimbursement for outstanding advances						$ 2,700,000								$ 2,700,000		2,700,000
Payments made to trusts by counterparty									71,400,000					181,400,000
Residential loans acquired by counterparty									63,800,000					159,300,000
REO acquired by counterparty									100,000					600,000
Gain recognized									$ 7,200,000					18,500,000
Obligation to reimburse amounts drawn on LOCs in excess of threshold						$ 17,000,000								$ 17,000,000		$ 17,000,000
Clean-Up Call Agreement Transactions [Member] | Maximum [Member]
Variable Interest Entity [Line Items]
Right to receive reimbursement for outstanding advances		$ 6,400,000