Investments, Summary of Investments (Details) - Securities with Maturity of One Year or Less [Member] $ in Thousands	Jun. 30, 2017 USD ($)
Summary of Investments [Abstract]
Cost	$ 43,250	[1]
Gross unrealized gains	0
Gross unrealized losses	(27)
Estimated fair value	43,223
Corporate Debt Securities [Member]
Summary of Investments [Abstract]
Cost	40,250	[1]
Gross unrealized gains	0
Gross unrealized losses	(25)
Estimated fair value	40,225
Other Municipal Debt Securities [Member]
Summary of Investments [Abstract]
Cost	3,000	[1]
Gross unrealized gains	0
Gross unrealized losses	(2)
Estimated fair value	$ 2,998

[1] Our available-for-sale securities are held at amortized cost.