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Income Taxes - Reconciliation of U.S. Federal Statutory Rate (Detail)
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Disclosure [Abstract]        
U.S. statutory rate (21.00%) (35.00%) (35.00%) (35.00%)
Change in valuation allowance   (116.90%) (5.40%) 14.90%
State income taxes, net of federal benefit   (5.20%) (1.20%) (0.70%)
Share-based compensation   8.30% 2.60% 2.70%
International tax rate differential   (6.70%) (6.40%) (5.50%)
Foreign dividend income   48.10% 1.00% 12.40%
Unremitted foreign earnings   (35.60%) 22.00%  
Foreign capital gain income       3.60%
Reserve for tax contingencies     (0.20%) 0.10%
Expiration of un-utilized tax credits   1.50%   10.40%
Unrealized gain on investments     30.70%  
Change in tax rate   52.40%    
Other   (2.60%) (1.50%) 1.20%
Income tax provision   (91.70%) 6.60% 4.10%