Marketable Securities and Equity Investments - Marketable securities and equity investments (Details) - USD ($)	1 Months Ended	3 Months Ended
	Jan. 31, 2018	Mar. 31, 2018	Mar. 31, 2017
Summary of cash equivalents and marketable securities
Other than temporary impairment losses, investments		$ 0	$ 0
Gross realized gains (losses)		0	0
Unrealized gain (loss) on investments		95,500,000
Payments to Acquire Investments		21,500,000	$ 0
CRISPR
Summary of cash equivalents and marketable securities
Unrealized gain (loss) on investments		92,500,000
Payments to Acquire Investments	$ 21,500,000
Moderna
Summary of cash equivalents and marketable securities
Unrealized gain (loss) on investments		2,900,000
Carrying value of investments		$ 23,000,000