Marketable Securities and Equity Investments - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Summary of cash, cash equivalents and marketable securities
Cash	$ 1,900,000,000		$ 1,900,000,000		$ 1,400,000,000
Other-than-temporary declines in fair value of available-for-sale debt securities	0	$ 0	0	$ 0
Gross realized gain (loss)	0	0	0	0
Unrealized gain (loss)	(31,200,000)	$ (61,200,000)	68,900,000	$ 88,200,000
Corporate equity securities
Summary of cash, cash equivalents and marketable securities
Fair value	236,184,000		236,184,000		$ 167,323,000
Other assets
Summary of cash, cash equivalents and marketable securities
Equity securities without readily determinable fair value, amount	$ 40,800,000		$ 40,800,000