Variable Interest Entities - Additional Information (Detail)	1 Months Ended	12 Months Ended
	Apr. 30, 2015 USD ($)	Dec. 31, 2016 USD ($) Trust	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Apr. 27, 2015 USD ($) Trust
Variable Interest Entity [Line Items]
Proceeds from sale of residual interests in Residual Trusts		$ 0	$ 189,513,000	$ 0
Mandatory cleanup calls required when loan pool falls to percent of original principal balance		10.00%
Variable Interest Entity, Primary Beneficiary [Member] | Residual Trusts [Member]
Variable Interest Entity [Line Items]
Number of securitization trusts in which residual interests are owned | Trust		5
Number of Residual Trusts sold | Trust					7
Proceeds from sale of residual interests in Residual Trusts	$ 189,500,000
Assets removed, deconsolidation of Residual Trusts					$ 783,900,000
Liabilities removed, deconsolidation of Residual Trusts					$ 588,500,000
Servicing rights capitalized upon deconsolidation of Residual Trusts	3,100,000
Loss on sale of Residual Trusts	$ 2,800,000
Variable Interest Entity, Primary Beneficiary [Member] | Non-Residual Trusts [Member]
Variable Interest Entity [Line Items]
Number of securitization trusts for which no residual interests are owned | Trust		10
Number of securitization trusts with reimbursement obligations | Trust		7
Variable Interest Entity, Primary Beneficiary [Member] | Non-Residual Trusts [Member] | Mandatory Clean-up Call For Residential Loans [Member]
Variable Interest Entity [Line Items]
Expected period to exercise mandatory clean-up call obligation		2017 and continuing through 2019
Total outstanding balance of residential loans expected to be called		$ 418,100,000
Variable Interest Entities Primary Beneficiary And Not Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Number of securitization trusts with reimbursement obligations | Trust		11
Reimbursement obligation on LOC, if drawn		$ 165,000,000
Amount available on LOCs for securitization trusts		$ 254,100,000	$ 260,400,000
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