Marketable Investment Securities, Restricted Cash and Cash Equivalents, and Other Investment Securities - Unrealized Gain (Losses) On Marketable Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Components of our available-for-sale investments
Marketable Investment Securities	$ 387,420	$ 417,094
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	4	96
Unrealized Losses		(1)
Unrealized Gains Losses, Net	4	95
Accumulated net unrealized losses
Accumulated net unrealized gain (loss), before tax, in accumulated other comprehensive income (loss)	1,000	1,000
Contractual maturities of restricted and non-restricted marketable investment securities
Debt securities with contractual maturities within one year	387,000
US Treasury and agency securities
Components of our available-for-sale investments
Marketable Investment Securities	65,992	10,016
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	3	32
Unrealized Gains Losses, Net	3	32
Commercial paper
Components of our available-for-sale investments
Marketable Investment Securities	298,435	369,397
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains		2
Unrealized Gains Losses, Net		2
Corporate securities
Components of our available-for-sale investments
Marketable Investment Securities	22,552	28,796
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	1	4
Unrealized Losses		(1)
Unrealized Gains Losses, Net	1	3
Other
Components of our available-for-sale investments
Marketable Investment Securities	$ 441	8,885
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains		58
Unrealized Gains Losses, Net		$ 58