Special Charges - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Employee	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Restructuring Cost and Reserve [Line Items]
Special charges recorded	$ 40,885,000	$ 10,445,000	$ 0
Charge to be paid in cash	48,400,000
Non-cash reduction to expense	7,500,000
Employee Severance
Restructuring Cost and Reserve [Line Items]
Special charges recorded	$ 23,500,000
Employee positions eliminated | Employee	255
Contract Termination
Restructuring Cost and Reserve [Line Items]
Special charges recorded	$ 14,400,000
Charge to be paid in cash	22,700,000
Non-cash reduction to expense	8,300,000
Other Restructuring
Restructuring Cost and Reserve [Line Items]
Special charges recorded	3,000,000
Non-cash reduction to expense	$ 800,000