Marketable Investment Securities, Restricted Cash and Cash Equivalents, and Other Investment Securities - Unrealized Gain (Losses) On Marketable Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Components of our available-for-sale investments
Debt securities	$ 872,878	$ 1,246,120
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	315	187
Unrealized Losses	(42)	(920)
Unrealized Gains Losses, Net	273	(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	868,000
Debt securities with contractual maturities extending longer than one year through and including five years	5,000
US Treasury and agency securities
Components of our available-for-sale investments
Debt securities	59,155	66,823
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	71	40
Unrealized Losses	(4)	(19)
Unrealized Gains Losses, Net	67	21
Commercial paper
Components of our available-for-sale investments
Debt securities	383,420	367,488
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Losses	(21)
Unrealized Gains Losses, Net	(21)
Corporate securities
Components of our available-for-sale investments
Debt securities	424,221	805,259
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	188	91
Unrealized Losses	(9)	(899)
Unrealized Gains Losses, Net	179	(808)
Other
Components of our available-for-sale investments
Debt securities	6,082	6,550
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	56	56
Unrealized Losses	(8)	(2)
Unrealized Gains Losses, Net	$ 48	$ 54