Marketable Securities and Equity Investments - Marketable securities and equity investments (Details) - USD ($)	1 Months Ended	3 Months Ended		6 Months Ended
	Jan. 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Summary of cash equivalents and marketable securities
Other than temporary impairment losses, investments			$ 0
Gross realized gains (losses)		$ 0	$ 0
Unrealized gain (loss) on investments		53,900,000		$ 149,400,000
Payments to Acquire Investments				21,500,000	$ 0
CRISPR
Summary of cash equivalents and marketable securities
Unrealized gain (loss) on investments		53,900,000		146,500,000
Payments to Acquire Investments	$ 21,500,000
Moderna
Summary of cash equivalents and marketable securities
Unrealized gain (loss) on investments		0		2,900,000
Carrying value of investments		$ 23,000,000		$ 23,000,000