Securitizations and Variable Interest Entities - Summary of Financial Asset and Liability Involvement with Consolidated VIEs (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Assets	$ 3,949,083	$ 4,640,831
Liabilities	(3,074,863)	(3,713,074)
Consolidated VIEs
Variable Interest Entity [Line Items]
Assets	1,364,440	1,670,067
Liabilities	(1,159,515)	(1,569,413)
Net Assets	204,925	100,654
Trust Certificates | Consolidated VIEs
Variable Interest Entity [Line Items]
Assets	913,387	1,226,957
Liabilities	(909,215)	(1,224,473)
Net Assets	4,172	2,484
Securitizations | Consolidated VIEs
Variable Interest Entity [Line Items]
Assets		375,607
Liabilities		(312,832)
Net Assets		62,775
Warehouse Credit Facilities | Consolidated VIEs
Variable Interest Entity [Line Items]
Assets	451,053	67,503
Liabilities	(250,300)	(32,108)
Net Assets	$ 200,753	$ 35,395