Marketable Securities and Equity Investments - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Summary of cash, cash equivalents and marketable securities
Cash	$ 1,700,000,000		$ 1,700,000,000		$ 1,400,000,000
Other-than-temporary declines in fair value of available-for-sale debt securities	0	$ 0	0	$ 0
Gross realized gains or losses	0	0	0	0
Unrealized gains to other (expense) income, net	56,500,000	$ 53,900,000	100,100,000	149,400,000
Payments to acquire investment			20,000,000	$ 21,500,000
Other assets
Summary of cash, cash equivalents and marketable securities
Equity securities without readily determinable fair value, amount	33,600,000		33,600,000
Corporate equity securities
Summary of cash, cash equivalents and marketable securities
Fair Value	$ 267,401,000		$ 267,401,000		$ 167,323,000