Business Combinations - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended	9 Months Ended
	Sep. 30, 2014	Jun. 30, 2014	Sep. 30, 2014
Business Acquisition [Line Items]
Expenses related to consulting service			$ 168
Clinical milestones [Member]
Business Acquisition [Line Items]
Contingent cash payments		20,100
Commercial Milestones Payments [Member]
Business Acquisition [Line Items]
Contingent cash payments		99,900
Additional shares relating to working capital adjustment		2
Pregenen [Member]
Business Acquisition [Line Items]
Acquisition completion date		Jun. 30, 2014
Shares of the company common stock		405
Cash paid to purchase of common stock		5,093
Common stock holding period after acquisition		18 months
Contingent cash payments		135,000
Consideration transferred to acquisition		19,348
Consideration transferred to acquisition, held back period		18 months
Business combination working capital adjustment		82
Discount rate applied to projected cash flows		15.50%
Gene editing platform expected useful life		8 years
Amortization expense	941		941
Deferred tax liabilities related to intangible asset		11,797
Transaction costs			171
Pregenen [Member] | Consultant [Member]
Business Acquisition [Line Items]
Issuance of stock in exchange of consulting services			3
Expenses related to consulting service			151
Pregenen [Member] | Upfront Payment [Member]
Business Acquisition [Line Items]
Consideration transferred to acquisition		15,636
Pregenen [Member] | Held back period of 18 months [Member]
Business Acquisition [Line Items]
Consideration transferred to acquisition		3,630
Pregenen [Member] | Preclinical milestones [Member]
Business Acquisition [Line Items]
Contingent cash payments		15,000
Pregenen [Member] | Commercial Milestones Payments [Member]
Business Acquisition [Line Items]
Contingent cash payments		$ 99,900
Pregenen [Member] | Additional shares of common stock [Member]
Business Acquisition [Line Items]
Shares of the company common stock		94