Commitments and Contingencies - Operating Leases (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) ft²	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2016 USD ($) ft²	Feb. 28, 2013 ft²
Operating Leased Assets [Line Items]
Operating leases, rent expense	$ 19.3	$ 18.5	$ 16.4
301 Binney Street | Biogen MA Inc.
Operating Leased Assets [Line Items]
Approximate Square Footage | ft²				79,683
Annual rental payments sublease	$ 6.1
West Kendall Sublease
Operating Leased Assets [Line Items]
Approximate Square Footage | ft²	78,454
Annual rental payments sublease	$ 4.8
320 Bent Sublease
Operating Leased Assets [Line Items]
Approximate Square Footage | ft²					104,678
Annual rental payments sublease	8.1
301 Binney Sublease
Operating Leased Assets [Line Items]
Approximate Square Footage | ft²					52,252
Annual rental payments sublease	$ 3.8
Tenant improvement allowance				$ 14.9