Income taxes - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Components Of Income Tax Expense Benefit [Line Items]
Income tax benefit (expense)	$ 612	$ (60)	$ 11,797
Effective income tax rate benefit	0.10%	0.00%	19.50%
Approximately valuation allowance increased	$ 105,300
Earlier Tax Year [Member]
Components Of Income Tax Expense Benefit [Line Items]
Federal, state and foreign income tax returns	Dec. 31, 2012
Latest Tax Year [Member]
Components Of Income Tax Expense Benefit [Line Items]
Federal, state and foreign income tax returns	Dec. 31, 2015
Research Tax Credit Carryforward [Member]
Components Of Income Tax Expense Benefit [Line Items]
Limitations on use of net operating losses and tax credit carryforwards, percentage	50.00%
Limitations on use of net operating losses and tax credit carryforwards, period	3 years
Significant accrued interest or penalties related to uncertain tax positions	$ 0	$ 0	$ 0
Interest or penalties Expense related to uncertain tax positions	0	0	0
U.S. [Member]
Components Of Income Tax Expense Benefit [Line Items]
Operating loss carryforwards	$ 466,800	$ 347,500	$ 130,000
Operating loss carryforwards expiration year	2036	2036	2036
Credit carryforward expiration year	2036	2036	2036
U.S. [Member] | Excess Equity Based Compensation Tax Deductions
Components Of Income Tax Expense Benefit [Line Items]
Operating loss carryforwards	$ 195,400
U.S. [Member] | Research Tax Credit Carryforward [Member]
Components Of Income Tax Expense Benefit [Line Items]
Tax credit carryforward amount	83,200	$ 44,900	$ 22,000
State and Local Jurisdiction [Member]
Components Of Income Tax Expense Benefit [Line Items]
Operating loss carryforwards	$ 456,800	$ 335,000	$ 115,500
Operating loss carryforwards expiration year	2036	2036	2036
Credit carryforward expiration year	2031	2031	2031
State and Local Jurisdiction [Member] | Excess Equity Based Compensation Tax Deductions
Components Of Income Tax Expense Benefit [Line Items]
Operating loss carryforwards	$ 195,400
State and Local Jurisdiction [Member] | Research Tax Credit Carryforward [Member]
Components Of Income Tax Expense Benefit [Line Items]
Tax credit carryforward amount	6,000	$ 3,800	$ 2,000
Foreign Tax Authority [member]
Components Of Income Tax Expense Benefit [Line Items]
Operating loss carryforwards	$ 0	$ 600	2,700
Excluding the impact of tax adjustments
Components Of Income Tax Expense Benefit [Line Items]
Income tax benefit (expense)			$ 11,800
Effective income tax rate benefit			0.00%