Investments in Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Current	$ 122,505	$ 88,072
Total Investments, Cost Basis	122,565	89,910
Total Investments, Accumulated Gross Unrealized Gains	33	388
Total Investments, Accumulated Gross Unrealized Losses	(93)	(89)
Total Investments	122,505	90,209
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total Investments, Current	122,505	88,072
Other Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Total Investments, Noncurrent	0	2,137
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Cost Basis	18,926
Debt Securities, Accumulated Gross Unrealized Gains	12
Debt Securities, Accumulated Gross Unrealized Losses	0
Debt Securities	18,938
U.S. Treasury Securities [Member] | Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Current	18,938
U.S. Treasury Securities [Member] | Other Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Noncurrent	0
Corporate Notes and Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Cost Basis	103,639	88,161
Debt Securities, Accumulated Gross Unrealized Gains	21	0
Debt Securities, Accumulated Gross Unrealized Losses	(93)	(89)
Debt Securities	103,567	88,072
Corporate Notes and Bonds [Member] | Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Current	103,567	88,072
Corporate Notes and Bonds [Member] | Other Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Noncurrent	$ 0	0
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities, Cost Basis		1,749
Equity Securities, Accumulated Gross Unrealized Gains		388
Equity Securities, Accumulated Gross Unrealized Losses		0
Equity securities		2,137
Equity Securities [Member] | Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities, Current		0
Equity Securities [Member] | Other Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities, Noncurrent		$ 2,137