Commitments and Contingencies - Additional Information (Detail)			1 Months Ended					3 Months Ended
		Jan. 29, 2018 USD ($)	Jul. 31, 2018 USD ($)	Apr. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Aug. 31, 2016 lawsuit	May 31, 2016 lawsuit	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	May 04, 2018 USD ($)
Commitments and Contingencies [Line Items]
Rental expense								$ 4,300,000		$ 3,800,000
Sublease rental income								77,000		0
Minimum rent payments								4,000,000		3,600,000
Security deposit - cash					$ 800,000			800,000
Security deposit - letters of credit					5,500,000			$ 5,500,000
Loan purchase obligation period								2 days
Contingent loan purchase commitment limit					53,200,000			$ 53,200,000	$ 54,200,000
Commitment repurchases								1,600,000		900,000
Deposit					9,000,000			9,000,000
Principal balance					17,300,000			17,300,000	20,100,000
Fair value					15,500,000			15,500,000	18,200,000
Pledged and restricted to support contingent obligation					2,200,000			2,200,000	2,200,000
Maximum exposure					6,000,000			6,000,000
Credit loss coverage reserve					0			0	0
Number of lawsuits | lawsuit						2	2
Preliminary litigation settlement awarded to other party		$ 125,000,000
Litigation settlement to be paid from insurance		$ 47,750,000
Litigation settlement paid by insurance					38,200,000			38,200,000
Contingent liabilities					101,889,000			101,889,000	129,887,000
Other assets	[1]				106,870,000			106,870,000	156,278,000
Regulatory litigation expense								13,500,000	77,250,000	$ 0
Subsequent Event
Commitments and Contingencies [Line Items]
Remaining limit of contingent loan purchase commitment											$ 0
Litigation settlement paid by insurance				$ 9,600,000
Litigation settlement paid by the Company			$ 62,500,000	$ 14,750,000
Insurance reimbursements receivable
Commitments and Contingencies [Line Items]
Other assets					$ 9,550,000			$ 9,550,000	$ 52,119,000

[1] Includes amounts in consolidated variable interest entities (VIEs) presented separately in the table below.