Commitments and Contingencies - Additional Information (Detail) ft² in Thousands			1 Months Ended	3 Months Ended		9 Months Ended
	Aug. 19, 2015 USD ($)	Apr. 16, 2015 renewal_option	Jun. 30, 2014	Sep. 30, 2015 USD ($) ft²	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) ft²	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Commitments and Contingencies [Line Items]
Lease agreement renewal term		5 years
Number of renewal terms | renewal_option		2
Rental expense				$ 2,000,000	$ 1,000,000	$ 5,000,000	$ 2,500,000
Annual lease payments				1,500,000	$ 900,000	4,100,000	$ 2,200,000
Security deposit made under lease agreement				700,000		700,000
Payments due in 2015				1,900,000		1,900,000
Payments due in 2016				11,400,000		11,400,000
Payments due in 2017				14,100,000		14,100,000
Payments due in 2018				15,100,000		15,100,000
Payments due in 2019				14,600,000		14,600,000
Payments due thereafter				71,100,000		71,100,000
Total payments due				128,200,000		128,200,000
Participation interests to be purchased			100.00%
Remaining limit of contingent loan purchase commitment				32,200,000		32,200,000
Maximum cash pledged				5,000,000		5,000,000
Pledged and restricted to support contingent obligation				3,400,000		$ 3,400,000		$ 3,400,000
Loan purchase obligation period						2 days
Contingent loan purchase commitment limit				$ 14,000,000		$ 14,000,000		$ 4,100,000
Springstone Financial, Llc
Commitments and Contingencies [Line Items]
Restitution	$ 700,000
San Francisco
Commitments and Contingencies [Line Items]
Area of property (square feet) | ft²				155		155
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		26