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2443405 - Disclosure - Income Taxes (Reconciliation of the Statutory U.S. Federal Income Tax Rate to Effective Income Tax Rate) (Details)
(http://www.alxn.com/role/IncomeTaxesReconciliationOfStatutoryUSFederalIncomeTaxRateToEffectiveIncomeTaxRateDetails)
Table(Implied)
Slicers (applies to each fact value in each table cell)
Reporting Entity [Axis]0000899866 (http://www.sec.gov/CIK)
Income Tax Disclosure [Abstract]Period [Axis]
2016-01-01 - 2016-12-31
2015-01-01 - 2015-12-31
2014-01-01 - 2014-12-31
Income Tax Disclosure [Abstract]
 
 
 
U.S. federal statutory tax rate
35.00%  
35.00%  
35.00%  
State and local income taxes
4.10%  
(0.80%) 
0.90%  
Foreign income tax rate differential
(33.80%) 
(32.50%) 
(16.50%) 
Tax credits, net of nondeductible expenses
(6.00%) 
(7.60%) 
(2.50%) 
Foreign income tax credits
(8.40%) 
(7.60%) 
(4.80%) 
Foreign income subject to U.S. taxation
26.60%  
24.30%  
15.80%  
Stock option compensation
   
U.S. deferred taxes on foreign earnings
16.50%  
60.10%  
0.00%  
Other permanent differences
(3.30%) 
0.10%  
(3.20%) 
Provision (benefit) attributable to valuation allowances
   
Effective income tax rate
30.70%  
 
71.00%  
 
24.70%