Marketable Investment Securities, Restricted Cash and Cash Equivalents, and Other Investment Securities - Unrealized Gains (Losses) On Marketable Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Accumulated net unrealized gains (losses)
Accumulated net unrealized gains, before tax, in accumulated other comprehensive income (loss)	$ (64)	$ (116)
Accumulated net unrealized gains, net of tax, in accumulated other comprehensive income (loss)	1,000
Components of available-for-sale investments
Debt securities	126,425	85,512
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	13	16
Unrealized Losses	(77)	(132)
Unrealized Gains Losses, Net	(64)	(116)
Contractual maturities of restricted and non-restricted marketable investment securities
Debt securities with contractual maturities within one year	119,000
Debt securities with contractual maturities extending longer than one year through and including five years	7,000
U.S. Treasury and agency securities
Accumulated net unrealized gains (losses)
Accumulated net unrealized gains, before tax, in accumulated other comprehensive income (loss)	(39)	(119)
Components of available-for-sale investments
Debt securities	75,638	81,982
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains		13
Unrealized Losses	(39)	(132)
Unrealized Gains Losses, Net	(39)	(119)
Commercial paper
Accumulated net unrealized gains (losses)
Accumulated net unrealized gains, before tax, in accumulated other comprehensive income (loss)	(1)
Components of available-for-sale investments
Debt securities	7,340
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Losses	(1)
Unrealized Gains Losses, Net	(1)
Corporate securities
Accumulated net unrealized gains (losses)
Accumulated net unrealized gains, before tax, in accumulated other comprehensive income (loss)	(24)	3
Components of available-for-sale investments
Debt securities	42,811	3,530
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	13	3
Unrealized Losses	(37)
Unrealized Gains Losses, Net	(24)	$ 3
Other (including restricted)
Components of available-for-sale investments
Debt securities	$ 636