Marketable Securities and Equity Investments - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Summary of cash, cash equivalents and marketable securities
Cash	$ 1,800,000,000		$ 2,300,000,000
Other-than-temporary declines in fair value of available-for-sale debt securities	0	$ 0
Fair value	165,178,000		282,084,000
Unrealized gain (loss)	(39,400,000)	$ 43,600,000
Equity Securities, Cost Basis	87,096,000		113,829,000
Other assets
Summary of cash, cash equivalents and marketable securities
Equity securities without readily determinable fair value, amount	46,600,000
Corporate equity securities
Summary of cash, cash equivalents and marketable securities
Fair value	165,200,000		$ 282,100,000
CRISPR Sale [Member]
Summary of cash, cash equivalents and marketable securities
Proceeds from Sale and Maturity of Marketable Securities	72,000,000.0
Equity Securities, Cost Basis	$ 26,700,000