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2452402 - Disclosure - Other Comprehensive Income and Accumulated Other Comprehensive Income (Schedule of Accumulated Other Comprehensive Income (Loss)) (Details)
(http://www.alxn.com/role/OtherComprehensiveIncomeAndAccumulatedOtherComprehensiveIncomeScheduleOfAccumulatedOtherComprehensiveIncomeLossDetails)
TableAccumulated Other Comprehensive Income (Loss) [Table]
Slicers (applies to each fact value in each table cell)
Reporting Entity [Axis]0000899866 (http://www.sec.gov/CIK)
Accumulated Other Comprehensive Income (Loss) [Line Items]Period [Axis]
2016-01-01 - 2016-12-31
2015-01-01 - 2015-12-31
2014-01-01 - 2014-12-31
Equity Components [Axis]Equity Components [Axis]Equity Components [Axis]
Accumulated Defined Benefit Plans Adjustment [Member]Accumulated Net Unrealized Investment Gain (Loss) [Member]Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]Accumulated Translation Adjustment [Member]Equity Component [Domain]Accumulated Defined Benefit Plans Adjustment [Member]Accumulated Net Unrealized Investment Gain (Loss) [Member]Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]Accumulated Translation Adjustment [Member]Equity Component [Domain]Accumulated Defined Benefit Plans Adjustment [Member]Accumulated Net Unrealized Investment Gain (Loss) [Member]Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]Accumulated Translation Adjustment [Member]Equity Component [Domain]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Accumulated comprehensive income (loss), Beginning balance
(10,000,000) 
(1,000,000) 
93,000,000  
(20,000,000) 
62,000,000  
(17,000,000) 
0  
87,000,000  
(14,000,000) 
56,000,000  
(12,000,000) 
0  
(4,000,000) 
(8,000,000) 
(24,000,000) 
Other Comprehensive Income before Reclassifications
2,000,000  
0  
46,000,000  
(4,000,000) 
44,000,000  
(2,000,000) 
(1,000,000) 
111,000,000  
(6,000,000) 
102,000,000  
(6,000,000) 
0  
110,000,000  
(6,000,000) 
98,000,000  
Amounts reclassified from other comprehensive income
1,000,000  
0  
(47,000,000) 
0  
(46,000,000) 
9,000,000  
0  
(105,000,000) 
0  
(96,000,000) 
1,000,000  
0  
(19,000,000) 
0  
(18,000,000) 
Other comprehensive (loss) income, net of tax
3,000,000  
 
0  
 
(1,000,000) 
 
(4,000,000) 
 
(2,000,000) 
 
7,000,000  
 
(1,000,000) 
 
6,000,000  
 
(6,000,000) 
 
6,000,000  
 
(5,000,000) 
 
0  
 
91,000,000  
 
(6,000,000) 
 
80,000,000  
 
Accumulated comprehensive income (loss), Ending balance
(7,000,000) 
 
(1,000,000) 
 
92,000,000  
 
(24,000,000) 
 
60,000,000  
 
(10,000,000) 
 
(1,000,000) 
 
93,000,000  
 
(20,000,000) 
 
62,000,000  
 
(17,000,000) 
 
0  
 
87,000,000  
 
(14,000,000) 
 
56,000,000