Liabilities Measured at Fair Value Using Level 3 Inputs (Detail) - Contingent Payment $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contingent acquisition consideration payable, Beginning balance	$ 85,609
Addition of contingent acquisition consideration payable related to the purchase of the Merck PKU Business	138,974
Changes in the fair value of contingent acquisition consideration payable for continuing development programs	6,825
Foreign exchange remeasurement of Euro denominated contingent acquisition consideration payable	(5,785)
Contingent acquisition consideration payable, Ending balance	161,637
Kyndrisa
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Reduction of fair value related to termination of development program	(43,652)
Reveglucosidase Alfa
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Reduction of fair value related to termination of development program	$ (20,334)