Investments in Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Current	$ 154,629	$ 88,072
Total Investments, Cost Basis	154,623	89,910
Total Investments, Accumulated Gross Unrealized Gains	58	388
Total Investments, Accumulated Gross Unrealized Losses	(52)	(89)
Total Investments	154,629	90,209
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Total Investments, Current	154,629	88,072
Other Assets
Schedule of Available-for-sale Securities [Line Items]
Total Investments, Noncurrent	0	2,137
U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Cost Basis	60,870
Debt Securities, Accumulated Gross Unrealized Gains	41
Debt Securities, Accumulated Gross Unrealized Losses	(15)
Debt Securities	60,896
U.S. treasury securities | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Current	60,896
U.S. treasury securities | Other Assets
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Noncurrent	0
Corporate notes and bonds
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Cost Basis	93,753	88,161
Debt Securities, Accumulated Gross Unrealized Gains	17	0
Debt Securities, Accumulated Gross Unrealized Losses	(37)	(89)
Debt Securities	93,733	88,072
Corporate notes and bonds | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Current	93,733	88,072
Corporate notes and bonds | Other Assets
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Noncurrent	$ 0	0
Equity securities
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 | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Equity Securities, Current		0
Equity securities | Other Assets
Schedule of Available-for-sale Securities [Line Items]
Equity Securities, Noncurrent		$ 2,137