Commitments and Contingencies - Additional Information (Detail) ft² in Thousands	1 Months Ended	2 Months Ended	3 Months Ended
	Apr. 30, 2015 ft² renewal_option	May. 16, 2016 USD ($)	Mar. 31, 2016 USD ($) ft² lawsuit	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)
Commitments and Contingencies [Line Items]
Lease agreement renewal term	5 years
Number of renewal terms | renewal_option	2
Rental expense			$ 3,200,000	$ 1,400,000
Minimum rent payments			2,200,000	$ 1,200,000
Security deposit - cash			800,000
Security deposit - letters of credit			4,700,000
Payments due in 2016			9,702,000
Payments due in 2017			15,090,000
Payments due in 2018			16,051,000
Payments due in 2019			15,619,000
Payments due in 2020			16,521,000
Payments due thereafter			57,190,000
Total payments due			$ 130,173,000
Loan purchase obligation period			2 days
Contingent loan purchase commitment limit			$ 180,700,000		$ 77,600,000
Commitment repurchases			3,800,000
Loan sales			15,113,000		$ 0
Deposit			9,000,000
Maximum cash pledged			5,000,000
Pledged and restricted to support contingent obligation			$ 3,400,000
Number of lawsuits | lawsuit			4
Subsequent Event
Commitments and Contingencies [Line Items]
Commitment repurchases		$ 22,300,000
Remaining Limit of Contingent Loan Purchase Commitment		$ 0
San Francisco
Commitments and Contingencies [Line Items]
Area of property (square feet) | ft²	112		169
Lease agreement expiration date			Jun. 01, 2022
Lease agreement renewal term			5 years
Westborough
Commitments and Contingencies [Line Items]
Area of property (square feet) | ft²			26