Marketable Investment Securities, Restricted Cash and Cash Equivalents, and Other Investment Securities - Unrealized Gains (Losses) On Marketable Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Accumulated net unrealized gains (losses)
Accumulated net unrealized gains, before tax, in accumulated other comprehensive income (loss)	$ (59)	$ (150)
Accumulated net unrealized gains, net of tax, in accumulated other comprehensive income (loss)	(1,000)	(1,000)
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains		22
Unrealized Losses	(59)	(172)
Unrealized Gains Losses, Net	(59)	(150)
Contractual maturities of restricted and non-restricted marketable investment securities
Debt securities with contractual maturities within one year	73,000
U.S. Treasury and agency securities
Accumulated net unrealized gains (losses)
Accumulated net unrealized gains, before tax, in accumulated other comprehensive income (loss)	(59)	(119)
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains		22
Unrealized Losses	(59)	(141)
Unrealized Gains Losses, Net	$ (59)	(119)
Commercial paper
Accumulated net unrealized gains (losses)
Accumulated net unrealized gains, before tax, in accumulated other comprehensive income (loss)		(2)
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Losses		(2)
Unrealized Gains Losses, Net		(2)
Corporate securities
Accumulated net unrealized gains (losses)
Accumulated net unrealized gains, before tax, in accumulated other comprehensive income (loss)		(29)
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Losses		(29)
Unrealized Gains Losses, Net		$ (29)