Fair Value and Balance Sheet Locations - (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivatives Fair Value [Line Items]
Derivative asset, fair value	$ 47,714	$ 18,101
Derivative liability, fair value	47,777	17,659
1.25% Call Option [Member]
Derivatives Fair Value [Line Items]
Derivative asset, fair value	46,578	17,080
Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member] | Prepaid Expenses and Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative asset, fair value	1,136	1,021
Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member] | Accrued expenses [Member]
Derivatives Fair Value [Line Items]
Derivative liability, fair value	0	0
Not Designated as Hedging Instrument [Member] | 1.25% Call Option [Member] | Other Assets [Member]
Derivatives Fair Value [Line Items]
Derivative asset, fair value	46,578	17,080
Not Designated as Hedging Instrument [Member] | 1.25% Embedded Cash Conversion Option [Member] | Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative liability, fair value	$ 47,777	$ 17,659