Restricted Investments, Held to Maturity (Details) $ in Thousands		3 Months Ended
		Mar. 31, 2019 USD ($) Securities	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) Securities
Schedule of Investments [Line Items]
Cost or Amortized Cost	[1]	$ 11,640		$ 17,413
Gross Unrealized Gains		2		1
Gross Unrealized Temporary Losses		(4)		(16)
Estimated Fair Value	[1]	$ 11,638		$ 17,398
Securities with unrealized losses for less than 12 months | Securities		12		20
Impairment losses		$ 0	$ 0
United States corporate securities [Member]
Schedule of Investments [Line Items]
Cost or Amortized Cost		11,540		$ 15,296
Gross Unrealized Gains		2		1
Gross Unrealized Temporary Losses		(4)		(16)
Estimated Fair Value		11,538		15,281
Municipal bonds [Member]
Schedule of Investments [Line Items]
Cost or Amortized Cost		100		1,082
Gross Unrealized Gains		0		0
Gross Unrealized Temporary Losses		0		0
Estimated Fair Value		$ 100		1,082
Negotiable certificate of deposits [Member]
Schedule of Investments [Line Items]
Cost or Amortized Cost				1,035
Gross Unrealized Gains				0
Gross Unrealized Temporary Losses				0
Estimated Fair Value				$ 1,035
Maximum [Member]
Schedule of Investments [Line Items]
Contractual maturities of fixed maturity securities		1 year
Duration of securities in unrealized loss position		12 months

[1] Refer to Note 5 for discussion about the differences between the carrying amounts and estimated fair values of the Company's restricted investments, held-to-maturity.