Marketable Investment Securities, Restricted Cash and Cash Equivalents, and Other Investment Securities - Unrealized Gain (Losses) On Marketable Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Components of our available-for-sale investments
Debt securities	$ 885,197	$ 1,246,120
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	369	187
Unrealized Losses	(7)	(920)
Unrealized Gains Losses, Net	362	(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	885,000
US Treasury and agency securities
Components of our available-for-sale investments
Debt securities	272,247	66,823
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	181	40
Unrealized Losses		(19)
Unrealized Gains Losses, Net	181	21
Commercial paper
Components of our available-for-sale investments
Debt securities	429,404	367,488
Corporate securities
Components of our available-for-sale investments
Debt securities	177,221	805,259
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	133	91
Unrealized Losses		(899)
Unrealized Gains Losses, Net	133	(808)
Other
Components of our available-for-sale investments
Debt securities	6,325	6,550
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	55	56
Unrealized Losses	(7)	(2)
Unrealized Gains Losses, Net	$ 48	$ 54