| Component: (Network and Table) | |
|---|---|
| Network | 153 - Disclosure - Effective Income Tax Rate (Detail) (http://www.alnylam.com/taxonomy/role/DisclosureEffectiveIncomeTaxRate) |
| Table | Reconciliation of Provision of Income Taxes [Table] |
| Reporting Entity [Axis] | 0001178670 (http://www.sec.gov/CIK) |
| Legal Entity [Axis] | Entity [Domain] |
| Reconciliation of Provision of Income Taxes [Line Items] | Period [Axis] | ||
|---|---|---|---|
2012-01-01 - 2012-12-31 | 2011-01-01 - 2011-12-31 | 2010-01-01 - 2010-12-31 | |
At U.S. federal statutory rate | .350 | .350 | .350 |
State taxes, net of federal effect | .049 | .046 | .042 |
Stock compensation | (.007) | (.033) | (.050) |
Other permanent items | (.016) | (.002) | .013 |
Valuation allowance | (.285) | (.361) | (.367) |
Effective income tax rate | .091 | (.012) | |