Consolidated Statements of Equity (Parenthetical) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Tax effect on unrealized gains (losses) recognized on securities available for sale	$ 133	$ 6,345	$ 4,136
Reclassification adjustment on securities available for sale	7,155	5,656	3,756
Tax effect on net unrealized gains (losses) recognized on securities available for sale	7,288	689	380
Tax effect of unrealized gains (losses) recognized on foreign currency forward contracts	3,647	1,268	254
Reclassification adjustment on foreign currency forward contracts	1,268	304	3,836
Tax effect of net unrealized gains (losses) recognized on foreign currency forward contracts	4,915	964	3,582
Tax effect of net unrealized gains (losses) recognized on pension benefit obligation	0	0	67
Accumulated other comprehensive income (loss)
Tax effect on unrealized gains (losses) recognized on securities available for sale	133	6,345	4,136
Reclassification adjustment on securities available for sale	7,155	5,656	3,756
Tax effect on net unrealized gains (losses) recognized on securities available for sale	7,288	689	380
Tax effect of unrealized gains (losses) recognized on foreign currency forward contracts	3,647	1,268	254
Reclassification adjustment on foreign currency forward contracts	1,268	304	3,836
Tax effect of net unrealized gains (losses) recognized on foreign currency forward contracts	4,915	964	3,582
Tax effect of net unrealized gains (losses) recognized on pension benefit obligation	0	0	67
Total Biogen Idec Inc. shareholders' equity
Tax effect on unrealized gains (losses) recognized on securities available for sale	133	6,345	4,136
Reclassification adjustment on securities available for sale	7,155	5,656	3,756
Tax effect on net unrealized gains (losses) recognized on securities available for sale	7,288	689	380
Tax effect of unrealized gains (losses) recognized on foreign currency forward contracts	3,647	1,268	254
Reclassification adjustment on foreign currency forward contracts	1,268	304	3,836
Tax effect of net unrealized gains (losses) recognized on foreign currency forward contracts	4,915	964	3,582
Tax effect of net unrealized gains (losses) recognized on pension benefit obligation	$ 0	$ 0	$ 67

Tax effect of the gross appreciation or the gross loss, net of reclassification adjustment, in the value of the total unsold available-for-sale securities at the end of an accounting period.

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Tax effects of the increase (decrease) to accumulated comprehensive income during the period related to benefit plans. While for technical reasons this element has no balance attribute, the default assumption is a credit balance consistent with its label.

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- Definition

Total tax effect of the change in accumulated gains and losses from derivative instruments designated and qualifying as the effective portion of cash flow hedges. The change includes an entity's share of an equity investee's increase or decrease in deferred hedging gains or losses. While for technical reasons this element has no balance attribute, the default assumption is a credit balance consistent with its label.

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- Definition

Tax effect on the reclassification adjustment for gains or losses realized upon the sale of securities during the period.

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- Definition

Tax effect on reclassification adjustment for accumulated gains and losses from derivative instrument designated and qualifying as the effective portion of cash flow hedges included in accumulated comprehensive income that was realized in net income during the period.

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- Definition

Tax effect on the change in accumulated gains and losses from derivative instruments designated and qualifying as the effective portion of cash flow hedges. Includes an entity's share of an equity investee's Increase or Decrease in deferred hedging gains or losses.

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- Definition

Tax effect on gross appreciation or the gross loss in value of the total of unsold securities during the period being reported on.

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