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Early Termination Charges - Additional Information (Detail) $ in Thousands	3 Months Ended		6 Months Ended	12 Months Ended
	Jun. 30, 2017 USD ($) Employee_Position	Mar. 31, 2017 USD ($)	Jun. 30, 2017 USD ($) Employee_Position	Dec. 31, 2017 USD ($)
Restructuring Cost and Reserve [Line Items]
Early termination charges	$ 2,262		$ 13,369
Headcount Reduction Plan [Member]
Restructuring Cost and Reserve [Line Items]
Cash payments				$ 31,000
Early termination charges	$ 2,262	$ 11,107
Number of employees elected to resign \| Employee_Position	352		352