Represents the Company's carrying value in equity securities categorized neither as held-to-maturity nor as trading.

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- Definition

The ownership percentage represented by the entity's equity investments in companies with which it conducts business and which are not accounted for under the equity method of accounting.

+ References

No definition available.

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- Definition

Accumulated appreciation or loss, net of tax, in value of the total of available-for-sale securities at the end of an accounting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-SubTopic 10

-Section 45

-Paragraph 11

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- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 323

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (a)(1)

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- Definition

The profit or loss on sales of previously unissued stock by subsidiaries or equity investees made to investors outside the consolidated group that is not reported as operating income. Represents the difference in the parent company's carrying amount of the equity interest in the subsidiary or equity investee immediately before and after the transaction.

+ References

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-Publisher FASB

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-Publisher SEC

-Name Regulation S-X (SX)

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- Definition

Amount of tax expense (benefit) before reclassification adjustments of unrealized holding gain (loss) on available-for-sale securities.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

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- Definition

Amount of consideration received by subsidiary or equity investee in exchange for shares of stock issued or sold. Includes amount of cash received, fair value of noncash assets received, and fair value of liabilities assumed by the investor.

+ References

No definition available.

+ Details

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Regulus - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended	12 Months Ended
Regulus - Additional Information (Detail) (USD $)	Mar. 31, 2009	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2014	Sep. 30, 2012
Schedule Of Equity Method Investments Joint Ventures [Line Items]
Company ownership percentage				12.00%	44.00%
Equity method investments	$ 10,000,000
Company ownership percentage		17.00%
Gain on issuance of stock by Regulus Therapeutics Inc.			16,084,000
Carrying value in Regulus Therapeutics Inc.		12,400,000alny_CarryingValueInEquitySecurities	12,400,000alny_CarryingValueInEquitySecurities	11,900,000alny_CarryingValueInEquitySecurities
Unrealized gain in other comprehensive income		15,700,000	15,700,000
Intraperiod tax benefit			(10,600,000)
Isis
Schedule Of Equity Method Investments Joint Ventures [Line Items]
Equity method investments	$ 10,000,000