Share-Based Payments (Tables)

12 Months Ended

Dec. 31, 2016

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)	2016			2015			2014
Research and development	$	84.5		$	88.6		$	102.1
Selling, general and administrative	121.7			127.3			150.3
Restructuring charges	(1.8		)	(8.6		)	—
Subtotal	204.4			207.3			252.4
Capitalized share-based compensation costs	(14.6		)	(11.0		)	(10.0		)
Share-based compensation expense included in total cost and expenses	189.8			196.3			242.4
Income tax effect	(54.0		)	(55.8		)	(72.2		)
Share-based compensation expense included in net income attributable to Biogen Inc.	$	135.8		$	140.5		$	170.2

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

	For the Years Ended December 31,
(In millions)	2016			2015			2014
Market stock units	$	38.4		$	38.1		$	37.4
Time-vested restricted stock units	120.0			119.0			115.4
Cash settled performance units	16.3			22.4			65.5
Performance units	18.6			13.9			21.9
Employee stock purchase plan	11.1			13.9			12.2
Subtotal	204.4			207.3			252.4
Capitalized share-based compensation costs	(14.6		)	(11.0		)	(10.0		)
Share-based compensation expense included in total cost and expenses	$	189.8		$	196.3		$	242.4

Stock option activity

	Shares		Weighted Average Exercise Price
Outstanding at December 31, 2015	107,000		$	53.94
Granted	—		$	—
Exercised	(41,000	)	$	53.75
Cancelled	—		$	—
Outstanding at December 31, 2016	66,000		$	54.06

Tax benefit and cash received from stock option exercises

	For the Years Ended December 31,
(In millions)	2016		2015		2014
Tax benefit realized for stock options	$	4.0	$	11.9	$	13.0
Cash received from the exercise of stock options	$	2.2	$	6.3	$	8.5

Market stock units activity

	Shares		Weighted Average Grant Date Fair Value
Unvested at December 31, 2015	269,000		$	339.89
Granted (a)	168,000		$	328.03
Vested	(155,000	)	$	244.68
Forfeited	(52,000	)	$	371.62
Unvested at December 31, 2016	230,000		$	355.60

Assumptions used in valuation of market based stock units

	For the Years Ended December 31,
	2016	2015	2014
Expected dividend yield	—%	—%	—%
Range of expected stock price volatility	38.2% - 40.7%	31.0% - 33.2%	31.7% - 35.1%
Range of risk-free interest rates	0.6% - 0.9%	0.2% - 1.0%	0.1% - 0.7%
30 calendar day average stock price on grant date	$260.67 - $304.86	$277.35 - $426.27	$280.88 - $335.65
Weighted-average per share grant date fair value	$328.03	$493.43	$395.22

Cash settled performance shares activity

	Shares
Unvested at December 31, 2015	192,000
Granted (a)	86,000
Vested	(117,000	)
Forfeited	(39,000	)
Unvested at December 31, 2016	122,000

Performance units activity

	Shares
Unvested at December 31, 2015	103,000
Granted (a)	55,000
Vested	(31,000	)
Forfeited	(17,000	)
Unvested at December 31, 2016	110,000

Time-vested restricted stock units activity

	Shares		Weighted Average Grant Date Fair Value
Unvested at December 31, 2015	810,000		$	323.87
Granted (a)	649,000		$	268.52
Vested	(406,000	)	$	285.13
Forfeited	(165,000	)	$	310.30
Unvested at December 31, 2016	888,000		$	303.49

Shares issued under employee stock purchase plan

	For the Years Ended December 31,
(In millions, except share amounts)	2016		2015		2014
Shares issued under the 2015 ESPP	190,000		78,000		**
Shares issued under the 1995 ESPP	—		98,000		180,000
Cash received under the 2015 ESPP	$	41.5	$	19.3	**
Cash received under the 1995 ESPP	$	—	$	30.0	$	46.4