Employee Benefit Plans	12 Months Ended
Dec. 31, 2014
Compensation and Retirement Disclosure [Abstract]
Employee Benefit Plans	Employee Benefit Plans We sponsor various retirement and pension plans. Our estimates of liabilities and expenses for these plans incorporate a number of assumptions, including expected rates of return on plan assets and interest rates used to discount future benefits. 401(k) Savings Plan We maintain a 401(k) Savings Plan which is available to substantially all regular employees in the U.S. over the age of 21. Participants may make voluntary contributions. We make matching contributions according to the 401(k) Savings Plan’s matching formula. All matching contributions and participant contributions vest immediately. The 401(k) Savings Plan also holds certain transition contributions on behalf of participants who previously participated in the Biogen, Inc. Retirement Plan. The expense related to our 401(k) Savings Plan primarily consists of our matching contributions. Expense related to our 401(k) Savings Plan totaled $49.3 million, $39.3 million and $32.8 million for the years ended December 31, 2014, 2013 and 2012, respectively. Deferred Compensation Plan We maintain a non-qualified deferred compensation plan, known as the Supplemental Savings Plan (SSP), which allows a select group of management employees in the U.S. to defer a portion of their compensation. The SSP also provides certain credits to highly compensated U.S. employees, which are paid by the company. These credits are known as the Restoration Match. The deferred compensation amounts are accrued when earned. Such deferred compensation is distributable in cash in accordance with the rules of the SSP. Deferred compensation amounts under such plan as of December 31, 2014 and 2013 totaled approximately $105.2 million and $84.7 million, respectively, and are included in other long-term liabilities within the accompanying consolidated balance sheets. The SSP also holds certain transition contributions on behalf of participants who previously participated in the Biogen, Inc. Retirement Plan. The Restoration Match and participant contributions vest immediately. Distributions to participants can be either in one lump sum payment or annual installments as elected by the participants. Pension Plans Our retiree benefit plans include defined benefit plans for employees in our affiliates in Switzerland and Germany as well as other insignificant defined benefit plans in certain other countries in which we maintain an operating presence. Our Swiss plan is a government-mandated retirement fund that provides employees with a minimum investment return. The minimum investment return is determined annually by Swiss government and was 1.75% in 2014 and 1.5% in 2013 and 2012, respectively. Under this plan, both we and certain of our employees with annual earnings in excess of government determined amounts are required to make contributions into a fund managed by an independent investment fiduciary. Employer contributions must be in an amount at least equal to the employee’s contribution. Minimum employee contributions are based on the respective employee’s age, salary, and gender. As of December 31, 2014 and 2013, the Plan had an unfunded net pension obligation of approximately $31.9 million and $22.6 million, respectively, and plan assets which totaled approximately $43.9 million and $38.1 million, respectively. In 2014, 2013 and 2012, we recognized expense totaling $9.8 million, $10.9 million and $3.8 million, respectively, related to our Swiss plan. The obligations under the German plan are unfunded and totaled $24.8 million and $20.0 million as of December 31, 2014 and 2013, respectively. Net periodic pension cost related to the German plan totaled $3.5 million, $3.3 million and $1.9 million for the years ended December 31, 2014, 2013 and 2012, respectively.

Employee Benefit Plans

We sponsor various retirement and pension plans. Our estimates of liabilities and expenses for these plans incorporate a number of assumptions, including expected rates of return on plan assets and interest rates used to discount future benefits.

401(k) Savings Plan

We maintain a 401(k) Savings Plan which is available to substantially all regular employees in the U.S. over the age of 21. Participants may make voluntary contributions. We make matching contributions according to the 401(k) Savings Plan’s matching formula. All matching contributions and participant contributions vest immediately. The 401(k) Savings Plan also holds certain transition contributions on behalf of participants who previously participated in the Biogen, Inc. Retirement Plan. The expense related to our 401(k) Savings Plan primarily consists of our matching contributions.

Expense related to our 401(k) Savings Plan totaled $49.3 million, $39.3 million and $32.8 million for the years ended December 31, 2014, 2013 and 2012, respectively.

Deferred Compensation Plan

We maintain a non-qualified deferred compensation plan, known as the Supplemental Savings Plan (SSP), which allows a select group of management employees in the U.S. to defer a portion of their compensation. The SSP also provides certain credits to highly compensated U.S. employees, which are paid by the company. These credits are known as the Restoration Match. The deferred compensation amounts are accrued when earned. Such deferred compensation is distributable in cash in accordance with the rules of the SSP. Deferred compensation amounts under such plan as of December 31, 2014 and 2013 totaled approximately $105.2 million and $84.7 million, respectively, and are included in other long-term liabilities within the accompanying consolidated balance sheets. The SSP also holds certain transition contributions on behalf of participants who previously participated in the Biogen, Inc. Retirement Plan. The Restoration Match and participant contributions vest immediately. Distributions to participants can be either in one lump sum payment or annual installments as elected by the participants.

Pension Plans

Our retiree benefit plans include defined benefit plans for employees in our affiliates in Switzerland and Germany as well as other insignificant defined benefit plans in certain other countries in which we maintain an operating presence.

Our Swiss plan is a government-mandated retirement fund that provides employees with a minimum investment return. The minimum investment return is determined annually by Swiss government and was 1.75% in 2014 and 1.5% in 2013 and 2012, respectively. Under this plan, both we and certain of our employees with annual earnings in excess of government determined amounts are required to make contributions into a fund managed by an independent investment fiduciary. Employer contributions must be in an amount at least equal to the employee’s contribution. Minimum employee contributions are based on the respective employee’s age, salary, and gender. As of December 31, 2014 and 2013, the Plan had an unfunded net pension obligation of approximately $31.9 million and $22.6 million, respectively, and plan assets which totaled approximately $43.9 million and $38.1 million, respectively. In 2014, 2013 and 2012, we recognized expense totaling $9.8 million, $10.9 million and $3.8 million, respectively, related to our Swiss plan.

The obligations under the German plan are unfunded and totaled $24.8 million and $20.0 million as of December 31, 2014 and 2013, respectively. Net periodic pension cost related to the German plan totaled $3.5 million, $3.3 million and $1.9 million for the years ended December 31, 2014, 2013 and 2012, respectively.