Marketable Investment Securities, Restricted Cash and Cash Equivalents, and Other Investment Securities - Unrealized Gains (Losses) On Marketable Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accumulated net unrealized gains (losses)
Accumulated net unrealized gains, before tax, in accumulated other comprehensive income (loss)	$ (150)	$ (116)
Accumulated net unrealized gains, net of tax, in accumulated other comprehensive income (loss)	1,000
Components of available-for-sale investments
Debt security	164,160	85,512
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	22	16
Unrealized Losses	(172)	(132)
Unrealized Gains Losses, Net	(150)	(116)
Contractual maturities of restricted and non-restricted marketable investment securities
Debt securities with contractual maturities within one year	164,000
U.S. Treasury and agency securities
Accumulated net unrealized gains (losses)
Accumulated net unrealized gains, before tax, in accumulated other comprehensive income (loss)	(119)	(119)
Components of available-for-sale investments
Debt security	74,788	81,982
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	22	13
Unrealized Losses	(141)	(132)
Unrealized Gains Losses, Net	(119)	(119)
Commercial paper
Accumulated net unrealized gains (losses)
Accumulated net unrealized gains, before tax, in accumulated other comprehensive income (loss)	(2)
Components of available-for-sale investments
Debt security	24,407
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)	3
Components of available-for-sale investments
Debt security	63,809	3,530
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains		3
Unrealized Losses	(29)
Unrealized Gains Losses, Net	(29)	$ 3
Other (including restricted)
Components of available-for-sale investments
Debt security	$ 1,156