Investments in Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Current	$ 198,788	$ 200,192
Total Investments, Cost Basis	199,832	204,441
Total Investments, Accumulated Gross Unrealized Gains	780	668
Total Investments, Accumulated Gross Unrealized Losses	(99)	(3,199)
Total Investments	200,513	201,910
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total Investments, Current	198,788	200,192
Other Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Total Investments, Noncurrent	1,725	1,718
US Government and Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Cost Basis	47,001	10,007
Debt Securities, Accumulated Gross Unrealized Gains	66	0
Debt Securities, Accumulated Gross Unrealized Losses	0	(12)
Debt Securities	47,067	9,995
US Government and Agency Securities [Member] | Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Current	47,067	9,995
US Government and Agency Securities [Member] | Other Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Noncurrent	0	0
Corporate Notes and Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Cost Basis	151,782	190,496
Debt Securities, Accumulated Gross Unrealized Gains	38	0
Debt Securities, Accumulated Gross Unrealized Losses	(99)	(299)
Debt Securities	151,721	190,197
Corporate Notes and Bonds [Member] | Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Current	151,721	190,197
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	1,049	3,938
Equity Securities, Accumulated Gross Unrealized Gains	676	668
Equity Securities, Accumulated Gross Unrealized Losses	0	(2,888)
Equity securities	1,725	1,718
Equity Securities [Member] | Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities, Current	0	0
Equity Securities [Member] | Other Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities, Noncurrent	$ 1,725	$ 1,718