Investments and Fair Value Measurements, Summary of Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Summary of Investments [Abstract]
Cost	$ 165,242	$ 202,100
Gross unrealized gains	3	0
Gross unrealized losses	(234)	(337)
Estimated fair value	165,011	201,763
Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	164,307	170,569
Gross unrealized gains	3	0
Gross unrealized losses	(225)	(265)
Estimated fair value	164,085	170,304
Securities with Maturity of One to Two Years [Member]
Summary of Investments [Abstract]
Cost	935	31,531
Gross unrealized gains	0	0
Gross unrealized losses	(9)	(72)
Estimated fair value	926	31,459
Corporate Debt Securities [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	82,414	132,434
Gross unrealized gains	0	0
Gross unrealized losses	(127)	(206)
Estimated fair value	82,287	132,228
Corporate Debt Securities [Member] | Securities with Maturity of One to Two Years [Member]
Summary of Investments [Abstract]
Cost	935	8,267
Gross unrealized gains	0	0
Gross unrealized losses	(9)	(35)
Estimated fair value	926	8,232
Debt Securities Issued by U.S. Government Agencies [Member] | Securities with Maturity of One Year or Less [Member]
Summary of Investments [Abstract]
Cost	81,893	38,135
Gross unrealized gains	3	0
Gross unrealized losses	(98)	(59)
Estimated fair value	81,798	38,076
Debt Securities Issued by U.S. Government Agencies [Member] | Securities with Maturity of One to Two Years [Member]
Summary of Investments [Abstract]
Cost	0	23,264
Gross unrealized gains	0	0
Gross unrealized losses	0	(37)
Estimated fair value	$ 0	$ 23,227