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, 2019	Dec. 31, 2018
Accumulated net unrealized gains (losses)
Accumulated net unrealized gains, before tax, in accumulated other comprehensive income (loss)	$ 174	$ (81)
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	174	51
Unrealized Losses		(132)
Unrealized Gains Losses, Net	174	(81)
Contractual maturities of restricted and non-restricted marketable investment securities
Debt securities with contractual maturities within one year	304,000
U.S. Treasury and agency securities
Accumulated net unrealized gains (losses)
Accumulated net unrealized gains, before tax, in accumulated other comprehensive income (loss)	134	21
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	134	40
Unrealized Losses		(19)
Unrealized Gains Losses, Net	134	21
Corporate securities
Accumulated net unrealized gains (losses)
Accumulated net unrealized gains, before tax, in accumulated other comprehensive income (loss)	40	(102)
Unrealized Gains (Losses) on Marketable Investment Securities
Unrealized Gains	40	11
Unrealized Losses		(113)
Unrealized Gains Losses, Net	$ 40	$ (102)