Collaboration Agreements (Adicet Bio and Decibel Therapeutics) (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended	12 Months Ended
	Jul. 31, 2016	Jul. 30, 2016	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Collaboration Agreements [Line Items]
Payments to Acquire Marketable Securities				$ 1,277,140	$ 809,419	$ 557,105
Adicet Bio [Member]
Collaboration Agreements [Line Items]
Period over which we have obligation		5 years
Upfront Payment Made	$ 25,000
Decibel Therapeutics [Member]
Collaboration Agreements [Line Items]
Upfront Payment Made			$ 25,000
Payment for preferred shares			$ 25,000