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Marketable Securities and Equity Investments - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Summary of cash, cash equivalents and marketable securities
Cash	$ 2,200,000,000		$ 2,200,000,000		$ 2,300,000,000
Other-than-temporary declines in fair value of available-for-sale debt securities	0	$ 0	0	$ 0
Fair value	312,682,000		312,682,000		282,084,000
Unrealized gain (loss)	69,800,000	$ (31,200,000)	102,300,000	68,900,000
Equity securities, cost basis	103,463,000		103,463,000		113,829,000
Other Assets
Summary of cash, cash equivalents and marketable securities
Equity securities without readily determinable fair value, amount	20,800,000		20,800,000
Corporate Equity Securities
Summary of cash, cash equivalents and marketable securities
Fair value	312,700,000		312,700,000		$ 282,100,000
Publically Traded Companies
Summary of cash, cash equivalents and marketable securities
Proceeds from sale and maturity of marketable securities			149,600,000	$ 0
Equity securities, cost basis	$ 51,300,000		$ 51,300,000