Marketable Investment Securities, Restricted Cash and Cash Equivalents, and Other Investment Securities - Investments Measured at Fair Value On A Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair value of marketable securities
Debt securities	$ 561,113	$ 417,094
US Treasury and agency securities
Fair value of marketable securities
Debt securities	5,011	10,016
Commercial paper
Fair value of marketable securities
Debt securities	484,969	369,397
Corporate securities
Fair value of marketable securities
Debt securities	69,194	28,796
Other
Fair value of marketable securities
Debt securities	1,939	8,885
Fair value measurements on recurring basis
Fair value of marketable securities
Cash Equivalents (including restricted)	2,808,795	2,436,545
Total	561,113	417,094
Fair value measurements on recurring basis | US Treasury and agency securities
Fair value of marketable securities
Debt securities	5,011	10,016
Fair value measurements on recurring basis | Commercial paper
Fair value of marketable securities
Debt securities	484,969	369,397
Fair value measurements on recurring basis | Corporate securities
Fair value of marketable securities
Debt securities	69,194	28,796
Fair value measurements on recurring basis | Other
Fair value of marketable securities
Debt securities	1,939	8,885
Fair value measurements on recurring basis | Level 1
Fair value of marketable securities
Cash Equivalents (including restricted)	840,775	246,876
Total	5,011	10,016
Fair value measurements on recurring basis | Level 1 | US Treasury and agency securities
Fair value of marketable securities
Debt securities	5,011	10,016
Fair value measurements on recurring basis | Level 2
Fair value of marketable securities
Cash Equivalents (including restricted)	1,968,020	2,189,669
Total	555,910	406,882
Fair value measurements on recurring basis | Level 2 | Commercial paper
Fair value of marketable securities
Debt securities	484,969	369,397
Fair value measurements on recurring basis | Level 2 | Corporate securities
Fair value of marketable securities
Debt securities	69,194	28,796
Fair value measurements on recurring basis | Level 2 | Other
Fair value of marketable securities
Debt securities	1,747	8,689
Fair value measurements on recurring basis | Level 3
Fair value of marketable securities
Total	192	196
Fair value measurements on recurring basis | Level 3 | Other
Fair value of marketable securities
Debt securities	$ 192	$ 196