Investments - Schedule of investments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt securities and restricted investments
Amortized Cost	$ 12,228	$ 13,285
Gross Unrealized Gains	6	1
Gross Unrealized (Losses)	(89)	(896)
Fair Value	12,145	12,390
Unrestricted available-for-sale, Debt securities
Debt securities and restricted investments
Amortized Cost	9,905	9,844
Gross Unrealized Gains	1	1
Gross Unrealized (Losses)	(89)	(887)
Fair Value	9,817	8,958
Unrestricted available-for-sale, Debt securities \| U.S. Treasury securities
Debt securities and restricted investments
Amortized Cost	6,978	6,936
Gross Unrealized Gains	1	1
Fair Value	6,979	6,937
Unrestricted available-for-sale, Debt securities \| Corporate debt securities
Debt securities and restricted investments
Amortized Cost	2,927	2,908
Gross Unrealized (Losses)	(89)	(887)
Fair Value	2,838	2,021
Restricted
Debt securities and restricted investments
Amortized Cost	2,323	3,441
Gross Unrealized Gains	5
Gross Unrealized (Losses)		(9)
Fair Value	2,328	3,432
Restricted \| Short-term bond
Debt securities and restricted investments
Amortized Cost	2,210	3,274
Gross Unrealized Gains	5
Gross Unrealized (Losses)		(9)
Fair Value	2,215	3,265
Restricted \| Money market fund
Debt securities and restricted investments
Amortized Cost	113	167
Fair Value	$ 113	$ 167

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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