Acquisitions - Allocation of Purchase Consideration Including Contingent Acquisition Consideration Payable (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Jan. 29, 2015
Business Acquisition [Line Items]
Goodwill	$ 197,039	$ 197,039
Prosensa Holding N.V
Business Acquisition [Line Items]
Cash and cash equivalents			$ 141,669
Trade accounts receivable			3,086
Other current assets			1,537
Property, plant and equipment			2,683
Intangible assets			497
Other assets			104
Acquired IPR&D			772,808
Total identifiable assets acquired			922,384
Accounts payable and accrued expenses			(68,799)
Debt assumed			(57,053)
Deferred tax liability			(193,202)
Total liabilities assumed			(319,054)
Net identifiable assets acquired			603,330
Goodwill			148,134
Net assets acquired			$ 751,464