Marketable Investment Securities, Restricted Cash and Cash Equivalents, and Other Investment Securities - Fair Value Measurements (Details) - Fair value measurements on recurring basis - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Fair value of marketable securities
Cash equivalents (including restricted)	$ 1,442,569	$ 1,278,971
Total	438,543	132,593
U.S. Treasury and agency securities
Fair value of marketable securities
Debt securities		22,476
Commercial paper
Fair value of marketable securities
Debt securities	345,252	101,959
Corporate securities
Fair value of marketable securities
Debt securities	92,768	8,068
Other
Fair value of marketable securities
Debt securities	523	90
Level 1
Fair value of marketable securities
Cash equivalents (including restricted)	155,933	172,025
Total		22,476
Level 1 | U.S. Treasury and agency securities
Fair value of marketable securities
Debt securities		22,476
Level 2
Fair value of marketable securities
Cash equivalents (including restricted)	1,286,636	1,106,946
Total	438,543	110,117
Level 2 | Commercial paper
Fair value of marketable securities
Debt securities	345,252	101,959
Level 2 | Corporate securities
Fair value of marketable securities
Debt securities	92,768	8,068
Level 2 | Other
Fair value of marketable securities
Debt securities	$ 523	$ 90