Marketable Investment Securities, Restricted Cash and Cash Equivalents, and Other Investment Securities - Unrealized Gain (Losses) On Marketable Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Components of our available-for-sale investments
Debt securities	$ 191,419	$ 1,246,120
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	157	187
Unrealized Losses	(8)	(920)
Unrealized Gains Losses, Net	149	(733)
Accumulated net unrealized losses
Accumulated net unrealized loss, before tax, in accumulated other comprehensive income (loss)	1,000	(1,000)
Accumulated net unrealized loss, net of 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	191,000
US Treasury and agency securities
Components of our available-for-sale investments
Debt securities	22,254	66,823
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	69	40
Unrealized Losses		(19)
Unrealized Gains Losses, Net	69	21
Commercial paper
Components of our available-for-sale investments
Debt securities	140,000	367,488
Corporate securities
Components of our available-for-sale investments
Debt securities	28,254	805,259
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	33	91
Unrealized Losses		(899)
Unrealized Gains Losses, Net	33	(808)
Other
Components of our available-for-sale investments
Debt securities	911	6,550
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	55	56
Unrealized Losses	(8)	(2)
Unrealized Gains Losses, Net	$ 47	$ 54