Commitments and Contingencies - Additional Information (Detail) ft² in Thousands			1 Months Ended	4 Months Ended	9 Months Ended	12 Months Ended
		Jan. 29, 2018 USD ($)	May 31, 2016 lawsuit	Aug. 31, 2016 lawsuit	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) ft² d renewal_option	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Feb. 21, 2018 USD ($)	Aug. 31, 2017 ft²	Apr. 30, 2015 ft²
Commitments and Contingencies [Line Items]
Lease agreement renewal term						5 years
Number of renewal terms | renewal_option						2
Rental expense						$ 15,700,000	$ 14,200,000	$ 7,400,000
Sublease income						400,000	0	0
Minimum rental expense						15,100,000	11,900,000	$ 6,000,000
Security deposit made under lease agreement, cash						800,000
Security deposit made under lease agreement						$ 5,500,000
Number of business days | d						2
Amount committed to purchase under the agreement						$ 54,200,000	32,200,000
Repurchased obligations						2,200,000	46,700,000
Investor agreement - purchases						26,700,000
Investor agreement - principal						20,100,000
Deposits to secure loans						9,000,000
Investor loan purchase agreement, fair value						$ 18,200,000
Credit support agreement, period						12 months
Credit Support Agreement, Investor Reimbursement					$ 13,000,000
Actual cash pledged and restricted to support contingent obligation						$ 2,200,000	3,400,000
Maximum exposure to loss						6,000,000
Credit support agreement, liability accrued for loss exposure						0
Number of claims filed | lawsuit				2
Accrued contingent liabilities						129,887,000	0
Other assets	[1]					$ 156,278,000	69,644,000
San Francisco
Commitments and Contingencies [Line Items]
Lease agreement, leased area (square feet) | ft²						169					112
Lease agreement expiration date						Jun. 30, 2022
Lease agreement renewal term						5 years
Westborough
Commitments and Contingencies [Line Items]
Lease agreement, leased area (square feet) | ft²						26
Lease agreement expiration date						Jul. 31, 2021
Subsequent Event
Commitments and Contingencies [Line Items]
Unfunded Loan Commitments									$ 0
Litigation settlement through second mediation		$ 125,000,000
Litigation settlement amount paid by insurance		$ 47,750,000
Amendment | San Francisco
Commitments and Contingencies [Line Items]
Lease agreement, leased area (square feet) | ft²										15
Lease agreement expiration date						Apr. 30, 2026
Federal and State Securities Class Actions
Commitments and Contingencies [Line Items]
Number of claims filed | lawsuit			2
Insurance reimbursement receivable
Commitments and Contingencies [Line Items]
Other assets						$ 52,119,000	$ 0

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