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, 2018	Dec. 31, 2017
Components of our available-for-sale investments
Marketable Investment Securities	$ 1,246,120	$ 479,812
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	187	80
Unrealized Losses	(920)	(276)
Unrealized Gains Losses, Net	(733)	(196)
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	1,040,000
Debt securities with contractual maturities extending longer than one year through and including five years	206,000
US Treasury and agency securities
Components of our available-for-sale investments
Marketable Investment Securities	66,823	84,286
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	40	22
Unrealized Losses	(19)	(141)
Unrealized Gains Losses, Net	21	(119)
Commercial paper
Components of our available-for-sale investments
Marketable Investment Securities	367,488	107,962
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Losses		(10)
Unrealized Gains Losses, Net		(10)
Corporate securities
Components of our available-for-sale investments
Marketable Investment Securities	805,259	282,256
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	91
Unrealized Losses	(899)	(124)
Unrealized Gains Losses, Net	(808)	(124)
Other
Components of our available-for-sale investments
Marketable Investment Securities	6,550	5,308
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	56	58
Unrealized Losses	(2)	(1)
Unrealized Gains Losses, Net	$ 54	$ 57