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Convertible Senior Notes - Interest Expense Recognized (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Debt Disclosure [Abstract]        
0.75% coupon $ 654 $ 647 $ 1,301 $ 1,287
Amortization of debt discount 3,355 3,249 6,755 6,420
Amortization of debt issuance costs 397 354 800 699
Total $ 4,406 $ 4,250 $ 8,856 $ 8,406
Interest rate of debt, stated percentage 0.75%   0.75%