Share-Based Payments (Tables)

12 Months Ended

Dec. 31, 2015

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Share-based compensation expense included in consolidated statements of income

	For the Years Ended December 31,
(In millions)	2015			2014			2013
Research and development	$	88.6		$	102.1		$	95.6
Selling, general and administrative	127.3			150.3			160.3
Reversal of previously accrued incentive compensation included in restructuring charges	(8.6		)	—			—
Subtotal	207.3			252.4			255.9
Capitalized share-based compensation costs	(11.0		)	(10.0		)	(9.8		)
Share-based compensation expense included in total cost and expenses	196.3			242.4			246.1
Income tax effect	(55.8		)	(72.2		)	(73.3		)
Share-based compensation expense included in net income attributable to Biogen Inc.	$	140.5		$	170.2		$	172.8

Summary of share-based compensation expense associated with each of our share-based compensating programs

	For the Years Ended December 31,
(In millions)	2015			2014			2013
Stock options	$	—		$	—		$	0.6
Market stock units	38.1			37.4			32.8
Time-vested restricted stock units	119.0			115.4			103.5
Cash settled performance units	22.4			65.5			109.8
Performance units	13.9			21.9			—
Employee stock purchase plan	13.9			12.2			9.2
Subtotal	207.3			252.4			255.9
Capitalized share-based compensation costs	(11.0		)	(10.0		)	(9.8		)
Share-based compensation expense included in total cost and expenses	$	196.3		$	242.4		$	246.1

Stock option activity

	Shares		Weighted Average Exercise Price
Outstanding at December 31, 2014	221,000		$	56.98
Granted	—		$	—
Exercised	(114,000	)	$	59.82
Cancelled	—		$	—
Outstanding at December 31, 2015	107,000		$	53.94

Tax benefit and cash received from stock option exercises

	For the Years Ended December 31,
(In millions)	2015		2014		2013
Tax benefit realized for stock options	$	11.9	$	13.0	$	29.4
Cash received from the exercise of stock options	$	6.3	$	8.5	$	28.1

Market stock units activity

	Shares		Weighted Average Grant Date Fair Value
Unvested at December 31, 2014	403,000		$	219.29
Granted (a)	185,000		$	493.43
Vested	(277,000	)	$	165.63
Forfeited	(42,000	)	$	294.85
Unvested at December 31, 2015	269,000		$	339.89

Assumptions used in valuation of market based stock units

	For the Years Ended December 31,
	2015	2014	2013
Expected dividend yield	—%	—%	—%
Range of expected stock price volatility	31.0% - 33.2%	31.7% - 35.1%	21.7% - 25.7%
Range of risk-free interest rates	0.2% - 1.0%	0.1% - 0.7%	0.1% - 0.7%
30 calendar day average stock price on grant date	$277.35 - $426.27	$280.88 - $335.65	**
60 calendar day average stock price on grant date	**	**	$150.33 - $240.14
Weighted-average per share grant date fair value	$493.43	$395.22	$193.45

Cash settled performance shares activity

	Shares
Unvested at December 31, 2014	335,000
Granted (a)	115,000
Vested	(222,000	)
Forfeited	(36,000	)
Unvested at December 31, 2015	192,000

Performance units activity

	Shares
Unvested at December 31, 2014	57,000
Granted (a)	89,000
Vested	(33,000	)
Forfeited	(10,000	)
Unvested at December 31, 2015	103,000

Time-vested restricted stock units activity

	Shares		Weighted Average Grant Date Fair Value
Unvested at December 31, 2014	1,137,000		$	221.01
Granted (a)	459,000		$	388.88
Vested	(626,000	)	$	190.65
Forfeited	(160,000	)	$	302.35
Unvested at December 31, 2015	810,000		$	323.87

Shares issued under employee stock purchase plan

	For the Years Ended December 31,
(In millions, except share amounts)	2015		2014		2013
Shares issued under the 2015 ESPP	78,000		**		**
Shares issued under the 1995 ESPP	98,000		180,000		245,000
Cash received under the 2015 ESPP	$	19.3	**		**
Cash received under the 1995 ESPP	$	30.0	$	46.4	$	38.7