Share-Based Payments (Tables)

12 Months Ended

Dec. 31, 2011

Share-based Payments [Abstract]

Share-based compensation expense included in consolidated statements of income

	For the Years Ended December 31,
(In millions)	2011			2010			2009
Research and development	$	62.0		$	62.7		$	60.8
Selling, general and administrative		88.7			123.6			106.4
Restructuring charges		(0.6	)		6.8			—
Subtotal		150.1			193.1			167.2
Capitalized share-based compensation costs		(4.5	)		(3.5	)		(6.3	)
Share-based compensation expense included in total cost and expenses		145.6			189.6			160.9
Income tax effect		(44.6	)		(60.3	)		(49.4	)
Share-based compensation expense included in net income attributable to Biogen Idec Inc.	$	101.0		$	129.3		$	111.5

Summary of share-based compensation expense associated with different programs

	For the Years Ended December 31,
(In millions)	2011			2010			2009
Stock options	$	5.9		$	26.1		$	21.6
Market stock units		14.6			10.0			—
Time-vested restricted stock units		89.6			129.4			133.7
Performance-vested restricted stock units settled in shares		1.0			5.3			4.6
Cash settled performance shares		32.7			15.0			—
Employee stock purchase plan		6.3			7.3			7.3
Subtotal		150.1			193.1			167.2
Capitalized share-based compensation costs		(4.5	)		(3.5	)		(6.3	)
Share-based compensation expense included in total cost and expenses	$	145.6		$	189.6		$	160.9

Weighted average assumptions to estimate fair value of stock option grants awarded

	For the Years Ended December 31,
	2011			2010			2009
Expected option life (in years)		*	*		4.5			4.7
Expected stock price volatility		*	*		30.8	%		39.3	%
Risk-free interest rate		*	*		2.0	%		1.9	%
Expected dividend yield		*	*		0.0	%		0.0	%
Per share grant-date fair value		*	*	$	16.52		$	18.00

Stock option activity

	Shares			Weighted Average Exercise Price
Outstanding at December 31, 2010		7,167,000		$	55.43
Granted		—		$	—
Exercised		(5,235,000	)	$	56.40
Cancelled		(241,000	)	$	53.30
Outstanding at December 31, 2011		1,691,000		$	52.75

Tax benefit and cash received from stock option

	For the Years Ended December 31,
(In millions)	2011		2010		2009
Tax benefit realized for stock options	$	47.5	$	16.0	$	1.5
Cash received from the exercise of stock options	$	291.9	$	160.0	$	25.2

Market Stock Units Activity

	Shares			Weighted Average Grant Date Fair Value
Unvested at December 31, 2010		398,000		$	61.87
Granted (a)		398,000		$	74.19
Vested		(119,000	)	$	59.89
Forfeited		(96,000	)	$	65.37
Unvested at December 31, 2011		581,000		$	69.49

Assumptions used in valuation of market based stock units

	For the Years Ended December 31,
	2011	2010
Expected dividend yield	0%	0%
Range of expected stock price volatility	25.7% - 33.4%	28.3% - 38.8%
Range of risk-free interest rates	0.3% - 1.9%	0.3% - 2.0%
60 calendar day average stock price on grant date	$66.78 - $101.16	$49.08 - $54.12
Weighted-average per share grant date fair value	$ 74.19	$ 61.87

Cash settled performance shares

(In thousands)	Shares
Unvested at December 31, 2010		370,000
Granted (a)		490,000
Vested		(187,000	)
Forfeited		(111,000	)
Unvested at December 31, 2011		562,000

Time-Vested Restricted Stock Units Activity

	Shares			Weighted Average Grant Date Fair Value
Unvested at December 31, 2010		4,223,000		$	53.26
Granted		1,398,000		$	70.01
Vested		(1,980,000	)	$	53.72
Forfeited		(717,000	)	$	54.20
Unvested at December 31, 2011		2,924,000		$	60.72

Performance-Vested Restricted Stock Units (PVRSUs) Activity

	Shares			Weighted Average Grant Date Fair Value
Unvested at December 31, 2010		154,000		$	49.24
Granted (a)		1,000		$	53.64
Vested		(75,000	)	$	49.51
Forfeited		(33,000	)	$	48.61
Unvested at December 31, 2011		47,000		$	49.34

Shares issued under employee stock purchase plan

	For the Years Ended December 31,
(In millions)	2011		2010		2009
Shares issued under ESPP		0.4		0.6		0.6
Cash received under ESPP	$	22.8	$	23.5	$	22.6