Condensed Consolidated Statements of Comprehensive Loss (Parenthetical) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2014	Mar. 31, 2013
Non-cash stock-based compensation expenses included in operating expenses are as follows:
Research and development	$ 3,681	$ 2,087
General and administrative	$ 1,910	$ 988

The aggregate amount of noncash, equity-based employee and non-employee remuneration classified as general and administrative expenses.

No definition available.

Name:	alny_NonCashStockBasedCompensationExpensesGeneralAndAdministrative
Namespace Prefix:	alny_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration